@Uniquelyme, a couple of things:
1. I think we've all heard the "you make me so tired" comment. Like anything else, sometimes it's probably true. Most times though, I think they just say it to either cut the visit short or get a bigger tip. Though I usually tip the girls better when they seem to enjoy it ('cause then I enjoy it more too. No one likes a whiner).
2. When it comes to FS, my contention has always been that men work harder than women. The onus is on the man to perform and we all know that IS work. In most cases, the woman is largely passive and can just take it. So, I think the encounter probably tires the man out more than the woman.
3. In many cases, some of us have to work afterwards. No matter how tired I might feel, a little food & caffeine usually perk me up again. It probably works for them too. Or, they can take a nap during the day (which many of them do). Wish I had that option.
4. Where I think the service takes its toll is in terms of soreness. But, not having a vagina, its hard for me to judge that. I would think proper lubrication is key though. There are probably other tricks-of-the-trade (pardon the pun) that the pros employ as well.
5. There is also the Yuk factor. On that issue, I would think a lot of these girls become experts in imagining they are somewhere else, or full blown out-of-body experience. After a while, they probably learn how to deal with it or go insane. If it's really that bad, they are free agents; so they can always go back to doing whatever they were doing before.
