Life Hacks/Rich's taboos and interests
Sexual history
Rich has an unspoken weakness for teen sex. His mind has a hard time differentiating between 16 year olds and 18 year olds, and he is actually more turned on by girls in that exact 3 year age range. (Shauna at 15 was seen as older at the time because it is the norm for black girls to look older than their actual age.)
It is very likely that the only reason Rich has not wound up sleeping with another older teen girl already is because he has Shauna satisfying his needs so well. Otherwise, he would probably seek a new partner and once again fall victim to his failure to discern age properly combined with his preference for younger girls.
Rich does not want to admit it, but he is actually turned on by Rahne and Charity, and will be turned on by Lexi as well if Xander changes his sex to become her. (He is turned on by young teens, but will not mistake them for adults the way he will with older teens.) It is for this reason actually that he tends to get mad at Rahne when she starts imitating her mother to him. There is no predicting what he might do if an older daughter who was more skillful were to sexually manipulate him.