crotch rub
Rubbing or grinding one's own crotch against an object (not another person), usually for the purpose of masturbation. Does not apply to situations in which the crotch rubbing is unintentional or is being done by one character to another (use crotch grab instead)
For rubbing against another person, use buttjob, frottage, grinding, penises touching, bulges touching, thigh sex, thigh straddling, or tribadism instead.