Is it o to bring a gf or boyfriend to a special occasion conerning your kids?

well me and my ex have been broken up for 7 months. he has another girlfriend who he lives wit and we have a 6 month old son together. our son's baptisim is in 2 weeks and i told him that i dont want here there and i dont see da need for her to be there. I think its a disrespect towards me and its a very uncomfortable situation because i dont like her and everytime i see them together it hurts very much. he says she is going cause thats his girl and there is nothing wrong wit her being there.. id like to kno wat people think i dont think she should be there am i being childdish or or am i right. She is not his mother and done see the why she has to be there just the thought of it stresses me out and has caused another arguement between please help me.. If she goes im takin her out or im walkin out