Asking a poll question to people (whether all genders or just one) is silly. It's an individual thing and not something that can be quantified as if "most women" say it for 1 particular reason or another. This applies the same to men, adults, teens or otherwise. Asking this question implies that you're trying to figure out what a large sector of the population does and is full of folly. Every person whether they fall into a group of one sort or another is an individual and does so for their own unique reasons.

If you're trying to determine why the female are with is sincere or not, you need to go to the source and ask her whether she means it or not. If you can't trust her words to mean what she says then why are you even with her?