There are a lot of accusations that feminism ruins dating and sex and whatever other thing insecure men (and women) have trouble doing anyways. There was a big deal over at Freethoughtblogs when youtube vlogger Thunderfoot was invited, decided to declare feminism as anti sex, insulted the network itself, and was promptly kicked off for being an ass. There was a big deal when bloggers declared harassment was an issue at atheist and skeptic conferences, with people claiming no it wasn't or that these bloggers hated men/sex/etc. There was a big deal when Rebecca Watson declared that it was awkward to be hit on in an elevator in the middle of the night and once more accusations that she hated men and sex.
Now I am pretty sure none of the people involved hate sex, most are self declared sex positive feminists, not some "radical" type that mostly live in the minds of the apparently socially awkward men and women. They, and most feminists I am sure, have no issues with flirting and sex, in the right situation. They only demand that people not reduce their entire being to sex, with their value determined solely by you desire to fuck them. Really it comes down to timing and social awareness. If the person is sitting in a booth answering questions about philosophy, science or similar, they probably don't want to be propositioned for sex. If they are in a bar flirting with you, they probably wouldn't mind as much, as long as the flirting is both ways. If someone is standing in a crowd, they probably wouldn't care to be grabbed or touched by someone they don't know.
If these seem obvious, congratulations you aren't a sexist pig. If, however, they seem hard to grasp or they make you angry and confused, you should probably not go into public until you have reassessed your ability to socialize. I personally have a few simple guidelines for when it is appropriate to tell someone they are attractive. Firstly, when you and the person in questions seem to be working towards a sexual relationship. That is, mutual flirting at the minimum and all the way up to relationships. Second, when it is directly relevant to an ongoing conversation, with you, from someone you know at least by name. Even then, there are smaller social cues that should tell you if it is appropriate, and which words and phrases to use. That is it. Any other situation is probably not ok because it implies the person you are "complimenting" only has value through attractiveness. This is really all feminists mean when they talk about objectifications. They don't mean you can never compliment looks, but that it shouldn't really be the only thing you see in a woman, or man, or any person. It should not be the only thing a person is measured by, end of story.
Media itself is still very sexist, portraying women not as attractive or intelligent but as pieces of sexual organs. Faces are often excluded and portrayals of women as obedient sex slaves are common. If this seems ok to you, you are sexist. If you treat women like this in real life, you are probably sexually harassing them. There is no one saying you can't have sex or flirt or anything of the sort. The people who think so can fuck off and get over themselves.