Whilst it's perfectly normal to notice and admire an attractive person, it's not nice or respectful (especially for the recipient of the gawking) to stare.
That said, if you had a good relationship and strong self esteem, I don't think it would bother you half as much as it does, the fact that it is making you miserable suggests a much deeper problem.
I'm afraid both DH and I were very guilty of gawking recently in Las Vegas, you just can't help it, practically all the waitresses in the casinos are gorgeous and they wear very little, when 6 of them fanned out in front of us, DH didn't know where to look first, his head was like an owls! I had a nice eyeful of some male eye-candy at the shows too.
If you are secure and happy in a relationship,with a partner who makes you feel attractive, window shopping isn't a problem - I'm a 44 year old , size 16 woman, no way can I compete with a 25 year old hottie with everything still pert and firm, I'd be far more offended if he tried to pretend he didn't notice them, he's a man, of course he noticed, if he didn't then I would point them out.
If he hadn't nudged me, I wouldn't have seen the two hot young male models with their tops off, showing off their buff young bodies in the entrance of A and F in New York, there was a crowd of middle aged women stood around with their tongues hanging out taking photos.
There's nothing wrong with noticing or an admiring glance - staring is very rude and disrespectful in a "normal" everyday situation though.