I know that this may sound strange, and I don't share this trait myself, but for many people sex is a critical part of forming a romantic bond with their partner. Without sex, they feel unloved and invalidated in the relationship. That's just part of their emotional needs. So for some people sex is indeed a psychological need.
But a large fraction of people would find their lives made much, much worse if they were unable to eat solid food, to leave their house, or to drive a car.
Exactly the same goes for sex.
Brief reminder of the context for this discussion: the person the OP is about had a child despite not having much money, various people here said you shouldn't be having children if you're too short of money, others pointed out that not having children is sometimes difficult, and that is when user rubidium suggested that total abstinence from sex might be the answer.
So we're talking here about whether it's reasonable to say that poor people should just never have sex.
I guess opinions on that might vary. My opinion is: duh, no, what an absolutely terrible idea. You don't literally have to have sex any more than you literally have to have friends or holidays or music, but just like any of those it's a hell of a thing to say whole classes of people should just do without.
You don't need to have sex. Abstinence is a form of birth control as much as being bald is a hairstyle.