They don't really forbid those two things. They just don't want their fellow jedi, going into darkside rage mode because their wife/husband was killed or their son/daughter was killed or what have you. So they fear the consequences of those things, making them have to forbid it.
Perfectly explained. So long as you can handle the death of your loved ones and don't mix love with lust, it's all right to love, marry, have sex, orgies and whatever.