Right heres my thoughts on the \"Marine Problem\"-
As others have said, in the strictly monaistic society of a space marine chapter, sex is simply inappropriote. As many well know, one of the core themes of marines is them giving up their humanity to protect the rest of the imperiums.
Secondly i daresay there are \"suppliments\" in the diet of a marine that would supress such needs and urges when they become over powering (god knows how Dante survived 1000 and still more years alive with no sex without some aid). In that way marines are not strictly \"sterile\" but that unfourtunate urge is less prevailent.
Thirdly, there are cases, particularly within the White Scars where marines live within the community and even are wed to women, so one must assume they are not so \"robotic\" that they are incapable of empathy or the basic masculine feelings of protection for other, less strong, human beings.
Then of course getting taken through puberty on 19 types of major surgery, religious indoctrination and ceaseless arms training can seriously mess with a young humans mind, its kinda like controlled child abuse so by the end of it a marine is a very disturbed, if beneficially so, individual.
But generally i would say that marines part and part through dietry suppliments and the strict training can control any urges for manual self gratification or gratification of any kind, that said they are not incapable of feeling for women, but all is under strict control
Dr Death