Some men masturbate frequently in favor of having sex simply because it is less physically exhausting and simple to do. Sometimes, whilst the female partner is perfectly happy to please their male counterpart, the male finds it easier to relieve themselves; this can be for many reasons, such as needing certain physical stimulation that only the male knows how to do, (sometimes it can be very difficult to describe to a partner, and it is easier for them to do it themselves) or sometimes it can be as simple having grown up desensitised by easy access to pornography, and having somewhat unrealistic expectations.
However, if you want to get more scientific, then it is quite a simple explanation. When a male orgasms, a huge amount of different endorphins are released into the brain, making the male feel sexually satisfied, and more often then not quite relaxed and happy. It is in human nature to try to replicate these feelings as much as one can; so it may simply come down to the idea that your partner enjoys orgasms so much, that he will take every quick opportunity to do so.
Please note; I am not making a generalisation of all males, this is simply a list of possible reasons.
Have Fun
Sincerely Sir