one of seven deadly sins

[BillGatesJR]

A friend of mine and I at work were having a discussion about various religious viewpoints (mostly differences between Jewish and Christian beliefs). Anyway, we were talking about the seven deadly sins. Lust was a confusing one for me. I knew what lust is, but the way he tried to explain it made it really confusing, because there are apparently so many things that can determine whether or not what you are doing is lusting. He said it is when you look at a woman and have a strong desire for sex with that woman. Now wait a second. So if you look at a woman who happens to be attractive and it causes an erection, that is lust? When you fantasize as you fap about girls at school or work, is that lust? Please explain.