Hi Angel. I see you just joined. Welcome. Great first post.
Actually, you can narrow the definition of adultery even further, if you go by the Bible's internal definition, and the meaning of the Hebrew. (Help me out, Mark, if I get this not-quite-right.) Anyway, I figure the Bible's definition wins over Webster's, ya know? *grin* It seems to work like this ...
The Hebrew word for adultery, as in "Thou shalt not commit ..." means "woman who breaks her covenant." and the man who helps her do so. It doesn't mean "someone outside your marriage", exactly, though that's not exactly right to do either. But if you commit adultery, the Bible condemns you to death. If a fella and an unattached gal hookup without benefit of marriage, the Bible normally condemns them to GET married. Not so bad, but still cause for serious consideration and skipping the "casual encounters" thing.
The neat thing about this understanding of the definition is that not only does it mean that the 7th commandment is not disallowing PM, but the 10th somewhat expands that accomodation. It says, "You shall not covet your neighbor's house. You shall not covet your neighbor's wife, or his manservant or maidservant, his ox or donkey, or anything that belongs to your neighbor."
Not how explicit that is! It doesn't say "your neighbor's spouse", which would seem more inclusive. But it is ok for an unmarried woman to "covet" her neighbor's husband to be a husband to her as well. Doesn't say, "your neighbor's unmarried daughter", or "your unmarried neighbor", or no fella could EVER get married. Just limits it to a woman who is in a covenant marriage already. Cool, huh? PM allowed inside the 10 Commandments!