This is complex. No it might never be the same again. But he is still the same stupid dough ball you decided to hook up with three years ago. What did you see in him then that attracted you to him? What are his good qualities? Three years is quite a long time to get to know someone. What hardships have you shared? What life experiences brought you closer? What made him bf material for you? Did you share the same outlooks, same views on life, money etc. Did you argue? Were you in love with him? You are both very young in life to be 'settling down' Are you besotted with him? No, maybe not.
A serial philanderer is generally always a serial philanderer. Men who have several girlfriends (or boyfriends) and often at the same time tend to be driven by something lower down in the physiology than the brain. They have even identified a gene which predisposes a man to continually search for new sexual conquests.
A drunken one night stand is nothing. It is just that and he didn't have to tell you. A sober planned and considered affair is very different to an act of stupid drunken debauchery. In either case it is the trust in the relationship that suffers.
You can't continue to use his drunken stupidity as a stick to beat him with and then hold that over him for the rest his life. Either forgive him totally and straight away (because you might need that same forgiveness one day if you mess up) or leave him and make a clean break. Yes his stress and emotional disarray will be significantly worse after having left Uni into work, but you are not responsible for his emotional pain after you have separated. You are responsible for creating continued and deliberate emotional pain and are now causing him to feel this. I'm not sure what you want to achieve after this?
What are you doing by shunning him totally? Are you punishing him? You can prolong his agony in the wilderness months of this relationship by withholding your love and attention. It matters not that you hate the face of the person he screwed. It could have been any convenient bolthole. What do you want to achieve by acting this way? Why don't you just leave him? There are some great men out there with whom you will probably find a greater bond. He can then find someone else to love instead of being held on a noose? If you are having so many doubts get out and leave this man alone and find someone else to be in a more sustainable relationship. He will be better for it if you do. You will be better off finding someone else to trust.
Maybe he has sensed that 'something' was stifling in the relationship, that control was an issue and that might have been a reason why let his hair down and strayed? If you decide to stay in the relationship you both have to talk this through if you are to understand each other's motives, behaviours and reactions. A good relationship has to have balance and generally be equal in the power sharing and reciprocation. Tolerance, understanding and being non judgemental go a long way if they are offered unconditionally. If there is little balance, and vindictiveness from both sides there will be huge resentments later down the line. The outcome is a prolonging of this agony which is destined to be a break up.