Trump and his supporters are misogynistic and support the rape and abuse of women. That’s why saying such things make him popular with them; they *want *women to be assaulted, to be raped, and to be killed.
It was a boast, not a “joke”, said because he knew boasting of assaulting women is something his followers want to hear and is something he is proud of.
Also, doing and saying awful things and claiming it’s a “joke” is standard far-right rhetorical tactics. The neo-Nazis do it all the time, it’s an explicit strategy of theirs. Never believe them.