You need to think about the reality and not the fantasy.
Women, even the biggest fucking whale of a slob, can get a shag any time they want. Sure, some hang around on dating or swingers sites, but they are outnumbered by men in vast numbers. As such, they can choose any guy they want and specify anything they like.
So, if you try your luck on one of these places then you really need to be an 8/9/10 to do ok. There are exceptions to this, obviously, but if you're bang average and think that a plethora of pussy is going to launch itself at you on a free site then I've got some magic beans to sell you.
If a guy has the spare time, set up a profile on fabswingers. I did it, no photo, no verification, no details other than my name. Within 24 hours the account had over 150 messages from all sorts of men - 90% looked like copy and paste messages.
Now imagine that with a real account. With photos etc. As you say they have their pick of the litter. Why would the fuck the bald middle aged guy or the skinny IT consultant on there when there is some personal trainer thats in the area and offering the same thing.
Do the same on pof and a dummy account, with an average 6/10 girl and you'll get a similar response.
Civvie sites, take time and you have to be pateint and courtious and invest time for a date. And even then no guarantees.
If your simply looking for a shag, it depends how you value your time. The hours of text tennis back and forth is boring to many and simply not an option to those in commited relationships, punting is a quicker cheaper alternative. The 10 hours of back and forth to even get a date, you'd be better working overtime and having a punt with a guarantee.
The likes of SA have worked for me recently. As I dont mind investing the time and quite like the chase. The women on there are open sexually as a rule and the very nature of the vast majority of messages they receive tends to make sex chat come up quickly. Ive had 'freebies' but ive had travel/accommodation costs and a couple of no shows.