trisher makes a good point. I never had a problem with my ex-husband's (male) partner attending family events, but my parents made a huge issue of it, as did 2 of my brothers, notably threatening me that they would all boycott my daughter's funeral if I didn't 'sort it out' and soon after, refusing to come to my eldest daughter's 21st birthday party if he was present! Different circumstances, but the same problem - losing sight of the needs of people at the centre! I cannot put into words how awful this was for me at the time
At the funeral, the partner, who had been with my ex for five years and was not involved in the break up of our marriage, travelled with us in the funeral car, but sat in the background in church. He didn't come to my daughter's 21st. I feel he showed great sensitivity to step back and leave us to it - it wasn't fair, or just, but he earned a lot of gratitude and respect from me and my children.
OP, you and your husband are the reason there is a christening and even though he is the one who is being completely unreasonable, deep down, I am sure your sons would want both of you there. You don't have to play 'happy families', but I think it is one of those times when graciousness is called for - a strength not a weakness.
If you decide, with your partner, that you are able to give ground this time, I think it will pave the way for smoother times in the future. You will have the moral highground where your ex is concerned. Could your partner come to the church with you even if he doesn't attend the bash afterwards? I hope that you can be united in the decisions that are being placed upon you.