Going onto the people who use Couchsurfing to hook-up, yes it happens. Is it looked down on? Not really unless that is your priority. The guys who only host girls are big indicators of these, but they still get girls asking to host, so, yeah, it does work. It’s normally easy to avoid these people by just reading their reviews and talking to them if that isn’t your cup of tea.
Some people also use Couchsurfing parties to try and hook-up. I have heard many girls at these parties complain of this. The parties are meant to have fun and exchange cultures, languages, and stories, with the free-thinking, love and adventure in Couchsurfing though, hooking-up happens and it’s not a bad thing, just don’t go looking for it. It’s never a hostile atmosphere though, so don’t let this scare you away, it’s simply a reminder that people flirt a lot at parties sometimes.
In terms of general safety, be aware of the culture you are in, be aware that there are different cultural norms where you may be and different understandings of personal space, flirting, and nudity. Safety is important, which means you should take the time to read a host’s references. I see negative references so sparingly that if I was thinking to host someone and saw a negative reference I would immediately want to turn it down. Your messages with the person can be just as revealing. Ask them about where you’ll be sleeping if you want to know specifics – they’ll never be offended.
I say to pay attention to cultural norms because, in the states, a stranger hosting you of the opposite sex sitting talking to you in their underwear could be seen as straightforwardly coming on to them and set off alarms. In Italy, this could be just them being comfortable in their own home and not caring. Just remember cultures and norms, pay attention to people’s actions, and have common sense and you’ll be fine. I have never felt unsafe. But, I am also a white American male. The worst I’ve ever heard is a German girl having to politely decline flirtation from a host and it was never brought up again.