Mind you, the stepson is being pretty goddamned disrespectful to her right now. I wouldn’t say it’s enough to out him, but him not merely avoiding the topic of her sexuality, but outright lying by saying that her partner is instead her cousin and roommate? Oh hell no. He’s expecting his stepmom, who basically raised him per the OP, to get herself back in the closet whenever he’s around.
“I’m surprised so many of you would betray your de facto son to a complete stranger.”
It partly depends whether you think its betrayal or reducing the damage from a huge mistake they’re making.
Otara
I would not lie to protect a loved one in this situation because I could not love someone willing to hurt an innocent person like this.
Unconditional love is for dogs.
Right and wrong has nothing to do with whether you know somebody or not.
What if the situation was exactly the same, only you were related to the girl instead of the gay guy? Would you tell her then? Does morality and ethics shift according to the proximity of a given individual to you personally? Why do you think it’s ok for people close to you to fuck over strangers? Why do you think it’s unethical to warn a stranger that she’s being fucked over?
You’re asking if I’d warn a loved one if I knew that she was being tricked into a scam marriage? Yes.
I don’t think it’s OK for people close to me to fuck over strangers.
I don’t think it’s unethical to warn a stranger that she’s being fucked over.
You just expressed disbelief and shock that anyone would try to warn a “complete stranger” that she was being fucked over. Now you’re changing your tune?
No, dear. The disbelief was around warning a complete stranger while outing a loved one who doesn’t want to be outed. Not just warning a complete stranger about some potential impending sadness.
IMO, soon-to-be daughter-in-law does not qualify as “complete stranger.”
While that may be true, people in this thread are suggesting that they would behave similarly even if the soon-to-be daughter-in-law WAS an actual stranger.
Maybe that’s the source of my disagreement. I see someone who is about to marry into the family to be on basically an equal footing, from a “I don’t rat out my family” standpoint, as someone who is already in the family. This person is going to be coming to Thanksgiving dinner, getting baby shower gifts from me, etc. She is effectively a family member. So knowing that her fiance is telling her a giant lie that is almost 100% likely to badly hurt her and fuck up her life in the future, I am inclined to let her know about this, even though the fiance is also a family member.
That’s why it was a mistake when I used the word “family member” upthread.
If and when my hypothetical son divorces the woman I’m not going to have any more contact with her. No more Thanksgiving dinners, no more vacations. The only thing tying her to me is my son. It’s the same reason I wouldn’t inform a friend’s spouse if that friend were cheating. We may act like family or friends, but without the friend to connect us, we aren’t.
If I were Hank I’d feel honor-bound to tell Dale but if I were a friend of Nancy’s I wouldn’t, for those who know what I mean. 
There is zero moral difference here. You are watching one person fuck over another person. It’s morally irrelevant whether the one doing the fucking is a loved one. It’s not ok to let loved ones ruin innocent people’s lives just because they are loved one. That is a fucked up, broken ethos. If you can’t see that, you need Jesus.
It’s bizarre that you think this is remotely morally relevant.
You sound like my SO. He knows that I won’t lie in court to protect him and would expect the same from him. We compromised, I just wouldn’t appear in court unless compelled to. (Another good reason for us to be able to get married, then it wouldn’t be an issue) 
And your grandkids. Here’s hoping one of them doesn’t find him in bed with ‘uncle’ Sam.
And all this may be much further down the road. How do you keep a straight face a year in? Five years? Ten? When he’s got a piece on the side?
Bizarre to you, maybe.
That you wouldn’t lie in court to save someone you supposedly love is alien to me.
I understand that. Like I said, I’m with someone that feels the same as you. Forgot to mention, the other side of our compromise is that he won’t appear in court for me because he knows I wouldn’t want him to lie. Not that we expect either situation to come up.
I have an Uncle Scotty who’s in his mid-thirties and is currently dating a 20-year old. He cheats on her. A lot. She has no idea. She thinks this guy is her prince charming. She thinks he loves her.
But I can keep a straight face just fine. His cheating doesn’t even come into mind while I’m with them. I think what he’s doing is rotten. But I wouldn’t betray him. I know, I know. I’m a monster. I’ve never cheated on anyone and don’t plan to but I’m glad that I have friends who wouldn’t betray me, who’d lie for me, who’d bury hookers with me (that last one was a joke), etc.
In a way I love my friends more than I think I could ever love a SO. I can see myself cheating on a future wife far easier than I can see myself betraying a friend.
OK, I think I’m starting to see the point here. Is the “outing” someone’s sexuality the part you all are objecting to? What if you just told the girl that he doesn’t really love her, and just wants a baby factory/promotion ticket? Would that be OK with you?
Well, does Brenda have a mother-son relationship with the man concerned?
Because if my son were in that position, then I’d:
Try to talk him out of it. It is a shitty thing to do even if he believes he can be monogamous with her. Hell, maybe she’d even marry him given his past and his vow of monogamy and being a good father; she should have the choice.
Point out that just having an engagement might get him off the DADT problem if he breaks the engagement off for some other reason than being gay. The prospective bride would be hurt, but not as much as if it happened several years down the line.
Suggest that he postpone the wedding for as long as possible.
This is the bit where I diverge from most people on here: I probably wouldn’t tell the prospective daughter-in-law because then the son would most likely end up being outed to the military: the former prospective daughter-in-law or her family members or friends would tell them. That would fuck the son’s life up.
Granted, marrying for cover purposes could fuck the son’s life up too, but that’s his decision, not mine. Telling his future wife would be my decision. And who knows? Maybe he’s not as straight as I think.
I’d prioritise my son’s welfare over the prospective daughter-in-law’s because he’s my son. It’s selfish, but that’s the way it is; I’d expect her family to do the same.
I would also, in Brenda’s position, want the son to admit that my cousin-room-mate was actually my partner, partly out of respect for me and partly because it would gauge the future wife’s opinions about same-sex relationships. It could help him make a decision about whether to continue with the sham marriage.
Rule #42: You are not Gibbs. 