Post Heresy Emperor's Children were suggested to indulge in a little raping IIRC, and unless Slannesh gave them all extra gifts, it may be physically possible for Space Marines to have sex. It may just be that they just have no sex drive because of all the bio-engineering and psycho-conditioning.
But I think GW has been very conservative in terms of sex in fluff, since a large, if not the majority, of its customers are too young for it so it would be irresponsible and unwise to go into that area too much.
But just because it isn't mentioned in the fluff does not necessarily mean it's not possible. It's not like GW has gone on record to say its impossible.
The whole subject has just been censored by the =I= instead.