OP, have you had unprotected sex with him since he had unprotected sex with OW? If so, that is really serious. He could have given you something horrendous like syphilis or HIV. I mean, I'm sure he hasn't, but that's not the point.
I'm assuming that he had unprotected sex if he went for STD tests. Which, by the way, I HIGHLY doubt that he has done. You absolutely need him to give you proof.
By having unprotected sex, he could have exposed you to the horror of OW becoming pregnant and therefore being in your life forever because the baby would be DD's sibling!!!!
If he really didn't use a condom, I would be thinking what that means in terms of personality traits apart from being unfaithful. Someone who puts their spouse's health and entire world and future at risk like that, on so many different levels, is immature, dim-witted, has issues with impulse control, doesn't think through the consequences of his actions, and is quite possibly a bad choice of life partner over and above the infidelity issue.
AND, she's a colleague! She could complain, especially if she's junior to him. She could say that she felt she had no choice or she would lose her job. So, let's count. He has exposed you to
- STDs, which can kill you or take your fertility or, in the case of syphilis, drive you mad.
- Pregnancy of OW and her being inserted into your life forever, without your consent, because the baby would be DD's sibling.
- Threatened your family's income because OW is a colleague.
- Threatened your family's income via having to pay child support because she got pregnant.
- Put your mental health at serious risk.
- Exposed you and your marriage to the possibility of a court case if she turns round and says it was non-consensual. (Scorned women have been known to do such things.)
He has invited an utter stranger into your marriage, potentially along with her germs, DNA (pregnancy), and craziness. And you never consented to any of that.
Even if, to him, it was a meaningless roll in the hay, his terrible lack of judgement and absence of integrity would really, really have me questioning whether this is a personality I want to go the distance with. I just think that what he did might speak of character issues that are about other things than fidelity. If he has judgement and impulse control this bad, and if he can't adequately think through the consequences of such abominable behaviour before doing it, what other scrapes might he get you both into? How else might he let you down? He seems to be of weak character, especially if he didn't use a condom, and weak people terrify me. Weakness can have the same effect as abject cruelty, even if the behaviour comes from a different place. An example would be when my sibling wouldn't come anywhere near our parents when they were terminally ill for a couple of years each. Even though, for much of that time, they could carry on almost as normal. I cannot, and don't want to, imagine the scale of their hurt. But she wasn't cruel, she was so weak that she could not cope with it. I'm scared for you that if you carry on with such a weak man who has such poor judgement and lack of impulse control that he might let you down in ways such as this.
So, in a weird sense, maybe the extra-marital sex is a red herring. Maybe you need to be thinking about what he did in terms of his wider personality. You know him, after all these years, so you should be able to put it all together pretty well. Does he have other disappointing character traits? Has he let you down in smaller ways? Can you usually count on him to do the right thing? Is he kind to people when there's no benefit to him? Does what he's done fit with other aspects of his personality? Have you noticed that he's selfish in other ways? Is he impulsive, irresponsible, and shows bad judgement in other areas of his life, too?
I'm so sorry, OP. You have a lot of thinking to do.
ETA: If this infidelity is, indeed, a sign of personality issues as described above, finding out about the infidelity could be one of the greatest gifts you've been given. A chance to see the real man behind the mask, perhaps, and a chance to get away before wasting the rest of your life with him.