Well tales about asking for proof of love were in vogue when I was 15 I think, and these were cautionary tales.
I'm not in her head, yet from what we know, she was pretty clear with him from the beginning: wait until she's ready and that's that. So either the guy can barely control his urges (very bad: many women will need much longer periods of abstinence e.g. around childbirth, and he is now doing pretty much all he can to coerce her short of actual rape), or he was immature enough to expect time limits where there were clearly none. Either way, Sandra has every reason to think that he cares more about his itchy dick than about her feelings.
It is also clearly stated that she thinks he might have never loved her. It is also only said he told her about his frustration (nothing about the degree of how he tries to coerce her). Men, at least the ones I've met, tend to get frustrated, when you keep postponing something (work, trips, whatever, sex in this case) without being clear. Basically he just wants a confirmation if it's going to happen or not. If he should keep on waiting.
Except that he is threatening to walk away.
Given his behavior, she is right to doubt it.
Perhaps the guy would be better off with a rubber doll, it wouldn't postpone.