Both can be true, however, as for sex leading to love there are a lot of people out there who use it to mislead people, into thinking they will love them if they have sex. This is why a lot of those types of relationships don't work out, one is typically using the other just for sex, stringing them along (usually its the guy stringing the girl along, but there are lots of women who do this to guys as well!). I voted "B" because this is most often true. Rarely sex leads to love in relationships but for the most part its love leading to sex.
Based on many conversations I've had with older people with experience I've learned that : love leads to sex. And many say it can go either way. The truth about "sex leads to love" is a false interpretation of love. Those people are not in love with the person, but with the sex. You see many people, mostly men, who stay in relationships just for the sex. Sex and love can be separated... Most women get attached to a man after sex, but it usually doesn't happen with men. So further i say love leads to sex. The other is just men believing they're in love with the women, when in reality they are just in love with the pleasure of the act.
Both are true. Sex can lead to love and love can lead to sex. It makes me think of a kiss that leads to more. If you've heard my favorite summer song called "Somethin' 'bout a Truck" by Kip Moore, you know what I'm talking about.
Nether of the above. People have sex because they want to. Love is an action not a feeling. People can have sex and not love them. And its possible to love a person without having sex. But sex will not keep you together. People who mixed these two up often deal with the problem the hard way.