I'm not the least bit surprised that heterosexual married women would have a very black and white view of prostitution, so I really don't see the value in arguing with them.
I guess their fear is that if the women genuinely do enjoy it occasionally, than the man (their boyfriend, son or husband) is genuinely cheating. Regardless of whether we enjoy it or not, I don't think any of us would do it for free.
Personally I always think it's hilarious when clients insinuate that we have a 'connection' and should go out on a 'date'. If I wanted to shag someone for free I'd much prefer a woman or my husband.
It is a shame that sex work is this big, black dirty secret between the sexes. I guess Mumsnet would be even more horrified if they knew the majority of gay, male porn stars are actually heterosexual men, some of them even have families!