Before anything else:
In my version of 40k: Yes.
Firstly, Their organs are altered throughout their initiation for optimum efficiency, so to be honest there are a few things that need to go. They don\'t need to reproduce, so strike 1. There are probably more efficient ways to produce testosterone, rather than that being a single function of several for testicles, so strike 2. And lastly (since for some reason i\'m using faint baseball terminology, you knew how many reasons i was going to give), power armour or no, it seems a little pointless to me to make a superhuman warrior that has a spot so likely to cause a lot of pain (Another reason that proves the female body is better designed

). Need i say strike 3?
On the last, yeah, you could argue that they could change it, but if you\'re changing them that much, see the previous points.
As to actual castration, i wouldn\'t expect it. I guess there are loads of rituals observed during initiation of varying pain and effort depending on the chapter, but with all these changes going on, chemical castration would be easier.
Besides, let\'s be honest, if we\'re going for purely functional, a penis isn\'t actually required either (oo, did i make some of you wince?). But it\'s considered \'rather important\'. Besides, look at Astartes armour - that\'s not too small a codpiece for an otherwise featureless part of the body...
Behaviour isn\'t an issue either.
Even assuming that you haven\'t got other organs producing testosterone (or an equivalent/improvement), castration later in life has little effect on aggression, sex drive etc.
And what effects may or may not remain can be reintroduced/eliminated using that extensive hypnotherapy all marines undergo.
Besides, they\'re meant to have quite astounding devotion, right? And let\'s not forget that, despite numerous worrying stories on teh intertron involving examples of our two favourite loyalist power-armoured fighting forces having an intimate moment or three together, space marines are kind of billed as warrior-monks. With my emphasis on the monk part. Devotion might just be put above biologics, and i would say that a couple of minutes with Mrs Palm and her five daughters would go against at least the spirit of celibacy if not the letter. (Also the above with certain parts changed, respectively Sisters of Battle, nuns, and your euphemism of choice)
Perhaps the Emperor\'s Children had the right idea after all...
Of course, if you
wanted to argue with me (*smiles sweetly*), i guess, since they are rather cooped up like Waffles says, they could get a little Classical. Warrior bonding and all...
Pff. What i say is what i\'ve ruminated upon. It is, as i said, what goes in my version of the 40k universe. Of course, it does take all the fun out of everything, doesn\'t it? Humour is sadly lacking. I guess it wouldn\'t be the same if in the grim darkness of the far future, there was not only war but also steamy Astartes-on-Astartes action...