I am no Lore expert, I assume there should be some proper females on their ships that they have sex with between worlds. They are probably educated females who are proper with class unlike most females today. They would have quarters on the ships separate from the Space Marine section. Space Marines probably have 18 hour training shifts a day and don't need much sleep, 5 hours to spend time with a female and then they sleep for 1. They would probably spend their days being educated and doing things that girls like to do. They would be like space nuns who would not remain virgins.