Sometimes people are led astray by temptation. Since monogamy isn't actually natural, it's human imposed state, your husband loves you - but hey some woman might have come on really strong with him. If she gave him an std she probably does it alot so he didn't stand a chance of resisting her seduction. Since men are supposedly mostly visual it's not really all that hard to snare us...
So ... it was probably a one night stand when his hormones took over. If George cloony or brad bit stripped down in front of you and started pressing your buttons, would you be able to resist?
So it's just that, hormones. His heart and mind belong to you. Don't stress it. It's not you. It's mother nature. It happened, it sucks, but it's in the past now.