If genders were dynamic then you would wind up with some very inconvenient time lord marriages.
Just changing ages would make marriages inconveniant. River Song (40ish) married to David Tennent (late 30s) was believable. The marriage to Matt Smith (20s) wasn't. I've even heard a theory where Susan (introduced to viewers as the Doctor's granddaughter) was really the elderly grey haired wife of the Doctor who regenerated into a teenager. Their relationship went sour after they got chaperoned by her school teachers for two years. When Susan fell in love with someone else the Doctor locked her out of the Tardis and left without her.