Is it okay for my boyfriend to touch my butt through my clothes?
Not to be rude but to prove a point: If some other guy put his hand on your butt, would you consider it a sexual advance even though you had jeans on and he was only touching the outside of the clothing? My guess is that you would probably deck the guy because you would rightly interpret his action as sexual. Therefore, the presence or absence of clothing is not what necessarily defines sexual touching. The location of the touch does make a difference.
There is a natural progression for sex. One of the primary starting points is touch. If you are committed to not having sex before marriage (as you ought to be committed), then the sensible thing is to not start the sequence that eventually results in sexual intercourse.