'The old definition was at least clearer before. (Sex by force with someone not your wife).'
The rape of a person requires that there is no consent, nor a belief of consent, in addition to the physical act. There is nothing in the law that explicitly states that only a man can commit rape.
Until 1991, Marriage was considered to grant consent, both to the husband and to the wife, so there could be no rape between a married couple.
Until 2001rape required the involvement of at least one man and one woman. Since 2001, rape requires the presence of a man and at least one other.
Also until 2001 it was considered to be nearly impossible to rape a man as an erection was considered to be an indication of consent as not having an erection would make penetration near impossible to achieve. I have not yet seen this corrected, although I suspect it has been considering the availability of things like Viagra.
'Surprise sex', or 'Sex by Surprise', can still be classified as rape if there was no evidence that the victim had given or intended to give consent, as there would be no grounds for the accused to claim there was a belief of consent. 'Surprise sex' is therefor considered a euphemism for rape.
Regretting having sex after the event still remains consensual at the time of the act, so no claim of rape can be considered.
I hope this clarifies things a little. You can look up the law on rape, of cause, and look into the various campaigns to get the law changed. Some of those changes even make sense.