It isn’t adultery, but it is treachery. Marriages are not only based on chastity, but on trust.
That doesn’t mean a person you’re dating owns you. It means that they should be able to expect that you are on the up and up. If you’re not exclusive, say so. If you are, talk to him or her about the conditions under which contact with members of the opposite sex are in keeping with the exclusive nature of your relationship, as you two understand it. I don’t mean you can’t have friends of the opposite sex or that you can’t define what your rules are for you, whether the person likes them or not. I mean that there are to be no surprises, no false impressions given…and no false impressions given to the others you spend time with, either!
The habits of courtship become the habits of marriage. Don’t even slightly get into the habit of “what she doesn’t know won’t hurt her” or “I will be the judge of whether it is wrong.” Your friendships with others should be discussed and carried out in a way that honors the relationship with the person to whom you are most committed. If you foresee so much as having a coffee with a potential rival, you should have discussed that ahead of time.
And please, none of this “she knows what I do and she’s a big girl.” Your “arrangements” should be respectful, not humiliating, regardless of what your significant other is willing to put up with.
As for fornication, your body is not your own. Do not forget that your relationship with God must be honored in these choices, too.