I'm not sure, but I also think that guys underestimate how much women think about sex. But I can't speak for the entire female gender. I, however, think about it almost all the time. It's not even thinking - it's just automatic reactions to seeing ANY guy. Sometimes it's almost subconsciously, like if I see a weird old man, I won't imagine sex with him - but my mind will go through, automatically, the process where it decides NOT to want sex with him.
If I see a kind of "my type" of man, this happens again. He's a possible option for sex.
If I see a VERY "my type" of man, I'd want him in bed without doubt. All the waiting, and emotions, and falling in loves comes with people you get to know, it doesn't come with strangers on a bus or the cashier. Which is why the sex you have with a random hottie won't even compare to the mind blowing sex (spiritually and physically) with someone you're in love with. And this type of amazing sex is hard to imagine for anyone who haven't truly been in love .
So I don't think anyone should be offended if their respective other enjoys the look of attractive men/women
It's just like looking at a flower field; it's there, it's obvious and it's beautiful.