That is indeed the issue. Girls can pick and choose good looking guys to have sex with them, without having to be all that attractive themselves... but getting a worthwhile partner is a whole different kettle of fish.
There are far more well-educated, attractive and intelligent women than men around these days. The competition for "high value" men is pretty intense (as witnessed by female strategy forum), because most women don't want a man of lower social standing than themselves. And a "high value" man isn't someone with six-pack necessarily - it's someone with a brain, an education and a decent job who can make a good partner in the long term.
I started waking up to this reality when young, attractive female colleagues of mine starting asking me for dating advice and how to tell the guys who were worth a damn from those who just wanted to get their leg over. I was floored at first, because I couldn't understand how it was possible for these women to have any difficulty finding a nice boyfriend... but it was apparently really tough.
It's like an alternate reality to me: when I was young, it was me asking female friends for advice on how to get a nice girlfriend. Now that I'm middle-aged and married, the tables have turned.