Men just want sex
Men are women are very different. Us women are all about love, we are always asking ourselves that big question "does he love me?". For women love is a near instantaneous thing that we suddenly fall into. We suddenly realise that we can not live without a guy.
For guys it is different, quite different. Love for a guy is a slow process, it does not happen over a few days. It is more of a gradual unnoticed transition from lust and 'sex on the brain' to love. This transition is so slow for guys, that most guys do not notice it has happened for quite some time.
Why men want sex
The fact is that men are wired for sex. A child for a woman means that they will have to look after it for many years. When a guy fathers a child there was no reason for a long term commitment before marriage was invented.
Every child a guy has is more of his genetic code passed on. The more children that he has, the more of his traits are passed along. This has left the modern man with one subconscious drive; The drive to have as much sex as possible. This is because sex is required to father as many children as possible.
How to keep a guy
So how is a girl to keep a guy from getting with other women? Us girls need to provide the guys with the environment that they subconscious desire. We need to keep them sexually satisfied. Of course you do not want to go and have sex with every guy. You just need to make sure that you have sex with the guy you want to keep.
If you want to do something else other than sex, remember the saying that "the key to a man's heart is through his stomach". Just make sure you cook a lot for him and he will never leave.