Painless red rash / small bumps on head / top of shaft of penis after sex. will last for a week and go if i don't have sex again / stay if i do.

Lichen planus: You are describing koebner phenomenon where trauma is the inducer in people prone to have the condition. The most likely diagnosis is lichen planus but psoriasis is a possibility. So rather than becoming celibate , i recommend applying otc Hydrocortisone ointment several hours before and several hours after intercourse and see if that helps.