I think regular DC Joker might not rape partly because several writers have shown him not as asexual, but being actively uncomfortable with the idea of sex, and wrote it as a rather believable result of his psychosis.
The thing is, most of Joker's multiple choice of origins show him as having had a wife, and that he wasn't born a monster or even molded into one while he was a child, but broke and became one as an adult.
Having grown up as a fairly normal person, it's likely he might see sex as a time of vulnerability and exposure, rather than an act of taking power. Thus, any humor he found in the thought of forcing himself on a woman might be balanced out by a revulsion or fear of making himself that vulnerable and open to someone he was victimizing.
Him being "above" rape is, in my opinion, ridiculous, as the above posters have said. However, him not doing it because he's afraid of being exposed and vulnerable to another person, especially to someone who might come back to fight him later or someone who might somehow remind him of his wife, that I find believable.