Actually this seems to be one of the better thought out threads.

I doubt GW will ever answer the question directly, so it's up to us to speculate.

My opinion, and take it for what it is, they can, just aren't supposed to have sex or form romantic attachments. But being human its bound to have happened in the past at least once or twice.

Space marines don't live in their armor 24/7 by BL fluff itself, and there isn't anything on what exactly the hormones do to them physically. I could be misremembering but i think that it said somewhere that the implants were supplemental, not replacements. So if that is the case then the flesh would be more than willing.

They are also at their core human, and humans' will form attachments. These attachments can take many forms, but the probability is high that at least in some case's it will be romantic. So the spirit can be willing as well.

The last obstacle is time and indoctrination.

Given the schedule given for a space marine's day in one of the books (can't remember which) they are kept busy almost every minute of the day. So sneaking off for sex isn't much of an option. But why keep them so busy and focused if they don't want to engage in proscribed activities. That and the whole space marine culture is built on an order of warrior monks. Given several real world examples, celibacy is expected but not always followed.

In a way Space Marines are rather tragic figures, forever forsaking love for duty.