People judge the sexuality of others for one or more of the following reasons:
1. They're judgmental (and often correspondingly hypocritical), so they feel the need to decide how everyone else should live. They've usually been taught to be this way by their religion, parents, or other social institutions.
2. They're highly insecure about their own sexuality and gain a perceived sense of emotional protection and/or "rightness" by transferring that insecurity onto the sexuality of others. Creating a sexual scapegoat somehow makes these insecure people feel better about themselves.
3. They are malleable sheep who'll believe anything their "leaders" tell them to believe. Often, this includes being told that it's good and proper to judge what other people do in bed. The problem here is that, as we've seen on countless occasions, the "leaders" are often engaged in the very sexual acts they're so quick to condemn.
4. They're busybodys who have far too much time on their hands, so they choose to spend their sad existence sticking their noses into the private lives of others.
Those are the main reasons why some individuals choose to judge the sexuality of other people. There's never a valid reason for it, but they often feel bound and determined to do it anyway.