I think the old adage "Girls mature faster than boys" comes into play here.
Commitment is a sign of maturity, and it just takes some of us longer. I know if a girl seems unsure of herself in bed, it gets distracting and starts to feel awkward.
I think back to the advice my dad once gave my sisters and me when he snuck us onto the "premium members only" tennis courts at this fancy resort at the beach.
Also, I've seen girls wear pearls and a nice blouse to trashy outdoor drinking events, or heels to sporting events. We Check Out Other Women (Even if We're in Relationships) Sorry, but we notice a hot girl any time any place.
Hopefully most of us don't make it obvious, but we check out a cutie when we can.
But I've learned not to make things worse by suggesting that she's PMSing. If a guy feels like he does not deserve a woman or is not good enough for her, he will become jealous easily and worried that he's going to lose her to another guy.
On the other hand, I admit that I have tried to make girls I'm interested in jealous on purpose before.