When you have sex with multiple partners/outside of marriage I think it begins to eat away at the meaning of sex altogether. In church a week or two ago our pastor went over the meaning of sex in marriage.
"Sexual pleasure and sexual climax get their final meaning from what they point to. They point to ecstasies that are unattainable and inconceivable in this life." (taken from sermon notes)
That is to say that sex is God's gift to a husband and wife to allow them a little bit of heaven on earth. Take the pleasure of sex and multiply it by a trillion and that is what heaven is. Sex is one of God's way of letting us know what we get to look forward to.
When sex becomes a past time or is had outside of a loving marriage it cheapens its meaning and turns it into another old and tired cliche. That isn't what God wants for us. He designed sex to be an intimate (in the physical and spiritual sense) experience shared between two people and you can't have that with just anyone. The kind of sex where you can also experience God's glory can only be found in the kind of relationship designed and blessed by God.
Spiritual meaning of sex aside, you run a higher risk of contracting stds, getting pregnant, and breaking your's and/or someone else's heart with casual sex.