mifta is right in what he says. however, as a general rule it wont make any difference whatsoever, ie if you masturbate a lot now it wont make any difference to you being able to make babies later in life.
as far as why a man would masturbate if he's in a relationship.... he does it because he needs/wants to. men are different to 'most' women and are aroused many times a day; obviously there are exceptions. why do a lot of women buy lots of shoes? because they can't help themselves! masturbating is great for men and women and it really does make you feel better and women should never criticise their partner for masturbating. if you think he shouldn't masturbate when he feels aroused, you should be prepared to have sex with him every time he is aroused, ie 6 times a day! it is all perfectly natural.
take care