My first thought was “she’s not using birth control.” Make sure you protect yourself unelss you want a child.
From your update, it sounds like she’s been dreaming of you as her back-up plan. If that’s not what you want to be, then you shoudl probably walk away fromt his situation.
If you do think there is a chance you might want kids down the road get some sperm banked. Then get the vasectomy.
At least then, if you do have a kid later on, it will be a choice that you’ve had time to think about rather than “oops, surprise, my biological clock was winding down so here I am with a surprise pregnancy!”
This chick sounds like she thinks of drachillix as “Plan B”. Basically, he’ll do until Mr. Right comes along. And until Mr. Right comes along, he’d better behave himself and be her boyfriend when she wants one, then disappear when she doesn’t want one. So no vasectomy allowed until Mr. Right comes along and she definitely doesn’t need a Plan B anymore.
Skyrocketed is a bit much. Her odds of having a baby with Down syndrome, the disorder most associated with advanced maternal age, increase from 0.1% at age 30 to 0.25% at age 35 to 1% at 40.
I disagree with the folks who think he’s her backup plan: to me it sounds like she’s panicky because drachillix is supposed to be The One (who’ll wait for her to get her shit together) and his plan is throwing hers off-kilter.
Hopefully part of her “getting her shit together” plan includes working on better communication skills. Stuff like learning how threatening to never speak to someone again is a poor relationship gambit, and opening up communication when a FWB relationship might be a candidate for becoming something more.
If you don’t wan to have a baby with her, get the vasectomy. If you think you might, hold off. I wouldn’t be impressed by her ultimatum, either - you really do sound like her “Plan B,” and that doesn’t sound like a situation to bring a kid into.
If you get the snip, but later want to raise a child, you could always rent one or lease one, or whatever it is they do these days. Jizz in a tube and freeze it, perhaps?