3 years to wait isnt 'passing by' if you have the rest of your life together - why is he in such a desperate rush - is he worried that unless he 'has you' by marrying then you might leave? Does he think that once he has 'got you' then you wont?
You say you can learn by the mistake of marrying, but getting out of a marriage, especially if he IS abusive, and especially if you do have a baby, is much MUCH harder than getting into it. From what you have said you will feel obliged to try hard to keep the marriage together, or for the child, mearly because you took that step of marriage
Plus its destructive, expensive and brings out the worst in people.
If you have been in a previous violent relationships have you both had councelling, or have you bonded to each other in a desperate codependant unit, without actually understanding what is good and healthy in a relationship, and growing in self confidence and self esteem? Have you had help to recognise what led you to be in such a relationship - why you didnt notioce/ignored the warning signs and why you didnt leave before it got violent/at the first signs of abuse (which probably were not physical)
It does sound as if you have just gone along with what he wants even if it isnt what you want.
What were the circumstances of him being single for 4 months - did she kick him out when she realised he had been cheating? How long were you together before you found out you were the other woman - how did he reconcile the lies and deceit to you? Did he take full ownership of the situation or did he blame/excuse his GF (they were not really together/not sleeping together/she was a bitch/ he didnt want to upset her) Unless he has dealt with the reasons that he felt entitled to have a girlfriend and someone extra on the side he runs the risk of allowing it to happen again because he wont notice that he is crossing boundaries.
I think you both need to do some serious talking and serious growing up before you start talking marriage and children