Exactly Proleworth 'sex positive feminism' is such a misnomer - they might as well call it 'ok with the objectification of women within a male dominated framework nonfeminism'.
(Twisty at I blame the Patriarchy has a good post on this.)
I always thought the pretty obvious difference between sex and other activities is that sex involves emotional and physical intimacy generally achieved by putting oneself in a position of vulnerability with another person you trust and want to do that with.
The kinda big obvious difference though is that sex can lead to pregnancy.
Internal damage and STD's seem quite important to me too.
If sex were like any other activity then the next time my accountant neighbour helps me out with my accounts instead of helping him with his English I'll give him a blow job. If he does my tax return I'll let him sodomise me (that way I can't get pregnant). No doubt my DH and his wife will be cool with that.
I would also say in answer to Carmen's question about sex as an activity is that whether or not you think it is different to other activities the fact is that it is the Patriarchy itself which tells us that is so (girls who like it = sluts, etc). Therefore within its own constructions of taboo, monogamy, marriage, etc, sex is considered to be different and it is hypocritical of the patriarchy to suddenly change all the rules when it suits. (And also very confusing and sending mixed and damaging messages to its citizens).
If sex were not emotionally, physically and chemically different to other activities then it would be OK to do it with children, rape would just be considered as plain assault, people who work in porn wouldn't end up drugging themselves and having PTSD.
I totally agree with the comments made by Dittany that commodifying sex leads us to all sorts of trouble.
Anyway I hate the pron industry because the vast vast majority of it is about exploiting, humiliating, degrading, damaging, hurting and frightening women so that other people can jerk off (either because they get off on seeing women hurt or because they haven't yet 'found their humanity').
I would suggest that anybody who thinks porn is just fine and dandy goes and spends a day on a set - there was a journalist who did this a while ago and came away traumatised (and he wasn't the one getting fucked).
For me the 'yes but some porn is good' line of argument sounds, in construction, very similar to the 'but some rape accusations are false' argument.