All are bad. It's not that he enjoyed sex with another person (he probably did before he met you), but that he (1) broke a promise (2) hid it from you (3) probably lied some more to cover it up. It's wrong on so many levels. He risks bringing home a STD to you, and he risks destroying your financial security if it gets the girl pregnant. He violated a trust you have in him, sometimes that trust never comes back. It brings into question how much he really loves you or perhaps if he understands what true love really is.
If he did it with someone he had emotional feelings for, don't be surprised if he doesn't fight to keep the relationship going. His heart may be elsewhere.
If it did it with someone he had no feelings or respect for, he may be able to cut her off easier but it does say something about him: he risked a good stable relationship for a few quickies? What kind of person is this?