Because heaven forbid that women gain some control over their bodies through exercise and fitness activities:
And what, exactly, is the problem with women-only gyms?
That is also an argument against
- women getting an education
- women voting
- women in the workplace
- women having access to contraception
- women having access to abortions
For the upholders of patriarchy, the institution of marriage must be really bad if it has to be based on all sorts on constraining social structures otherwise, women would (1) not get married in the first place, and (2) leave it in a heartbeat if they had a choice.