I do expect sheets to be changed between occupants. If I wouldn’t sleep with someone, I wouldn’t typically sleep on the same sheets as they do. That seems like housekeeping 101 to me.
When I was a kid, my mom drilled it into my head that if I slept in someone’s guest bed, I must, must, must strip the bed on the last night, because they would be changing the sheets and I should do that to be helpful.
If changing sheets is the issue, just buy more sheets. They aren’t that expensive.
I completely understand your aversion to hot bunking. This is one of the areas that is difficult for a step-parent - type situation. You sometimes have to get . . . kind of intimate (not sexually, of course) with the kids before you really get to know them and bond with them.
It’s weird and I don’t know what to tell you. For me it’s not a germ thing, it’s just an “overshare” deal I guess.
I have a 3 year old. I carried her, birthed her and breast fed her. She drinks out of my drink? It is now her drink. I’ve always been skeeved out by a kid sharing a drink. I was told when it was my kid, I’d get over it. Nope. Sure haven’t.
Sometimes things just bug you. Maybe you could get the kid a toddler/racecar bed? At that age, he shouldn’t be too tall for it. Maybe a trundle bed?
Get a trundle bed for them. The little kid would love to sleep on a bed that comes out from under another bed.
Just a WAG, but could this have something to do with the fact that he’s another woman’s kid, and not your own? Would you feel the same way if he were yours?
I would not be offended by it were in in your SO’s place; but I would have to give some thought to whether you are really a serious candidate for a long term relationship if you have a very strong aversion to sleeping in the same space my child has been in, simply because he has been there at some time. This implies that any time I want to be with you in this way, it will either have to be in some other place or you will be unavoidably uncomfortable which means I will be uncomfortable.
Since the chances for our sharing this experience are already apparently limited to times when the child is not around (which is roughly half the time) this would rapidly become wearying to me. I myself have little tolerance for drama so I would be inclined to conclude that this relationship is unlikely to work out in the long run, for either one of us.
This. I don’t see the need to look for a deeper psychological problem, such as disliking the kid for having a different mother or unconsciously resenting him or whatever.
I would not want to sleep in sheets anyone else had slept in, except my own self or my SO. It’s not a ‘scarred psyche’ thing, it’s a hygiene thing. I would second the suggestion of having a second set of sheets. You can switch the sheet sets off and on without actually washing them every time, just wash both sets on laundry day.
If repeatedly changing the bed throughout the week is too much of a PITA, and for me it might be, then I think you have to evaluate just how objectionable the kid dirt may be.
Would you feel better if you had your own pillow? I can see not wanting to put your head where he might have drooled, but otherwise it’s not like you don’t know where he’s been.
This is something you need to talk out with your boyfriend. IMHO this appears to be a psychological block on your part and you’re not going to get anywhere without talking to him about it.
You know, sometimes a cigar is just a cigar. Lots of people are uncomfortable sleeping in sheets that someone else has slept in, with no psychological baggage involved. Try the easy solution of separate sheets before getting into Deep Conversations of Our Relationship and Past Child Abuse.
Hey, do you know how your boyfriend generally regards his kid’s cleanliness? Some parents would be offended at the idea that you think their precious snowflake has cooties, but others themselves think kids are basically grubby germy creatures and would understand your hesitancy.
I like the term “overshare”. I don’t think I’d be comfortable either. Sharing the same intimate space would make me aware of the child at the wrong times. Kid space is kid space, sexy time space is sexy time space. Unless you’re married or in a committed long-term relationship. Just dating - back to the word “overshare”.
Just for a chuckle… Easily grossed out? You’re more likely a conservative,
says Cornell psychologist.
You are being completely ridiculous and i would be incredibly offended.
How about looking at this from a different angle? If I was the child’s mother, I would not be happy that my child was sleeping on the same sheets that his father and his girlfriend were possibly intimate on.
If they’re uncomfortable sleeping on clean sheets, then yes, it’s entirely psychological. Perhaps not rejection or abuse issues, but germbophobia at least. There’s no reasonable health risk from sleeping on clean, nonsmelly sheets previously occupied by a healthy kid.
Besides, the OP says in the OP that “it just psychologically grosses me out”. 
I can understand why the OP is skeeved out by this. Sleeping space is very intimate - possibly sexual and certainly intimate.
Grown people whould not be intimate or sexual with children, especially children they aren’t parenting. And to be in this intimate child-space **should **cross a mental boundary. Bed=sex=child-I’m-not-related-to as a no-fly zone seems really really healthy to me.
I also agree on the germ-factor; I won’t sleep in a bed that has been occupied by someone who lies outside my comfort zone. I’d sleep in a bed occupied by my husband, kids or own mom, but not my sisters, their kids, or my dad. Go figure.
Anyway I think OP is totally normal and I second the suggestion of an alternate set of sheets and pillowcases. That way the kid’s drool and the couple’s…whatever will never meet.
Given this, I’d say that the issue is more with you than with your partner or his son - but I wouldn’t say that you were crazy or anything. 
If you were squicked out about having sex on the same sheets that he would later, or already had, slept on with his son, that would be completely and utterly understandable, and your partner should either change the sheets for you or help you do so.
If, as it seems, it’s just that you don’t want to share sheets with a five-year-old boy (which I can kinda understand; there’s no logical reason behind it, but I understand all the same), then you should be upfront about it and offer to wash the sheets yourself. Just be honest and say that you’re not accustomed to sharing beds with children, so you’re prepared to do extra laundry to avoid that. Of course, your BF might offer to wash the sheets more often than otherwise just because you ask, and that would be wonderful - but if he doesn’t, it’s no shame on him.
After a while you might not be so bothered. Hell, give it a year or three and it won’t happen that often anyway - the little boy will need his own bed. This is not a permanent problem.
He washes them weekly, not between each visit. So they’re not clean - they’ve been slept on by the kid and are unwashed. An aversion to that doesn’t necessarily have anything to do with who the previous sleeper is - hotbunking on someone else’s sheets, even if they’ve only spent one night on them, is just oogy. I’ll maybe accept germophobia, but all I’m saying is she should try the dedicated sheets solution before getting into Deep Issues With Rejection And/Or Abuse.
Weird. Very weird. Get over it.