There is something which is not clear to me.
Is the main point of this family party to welcome/introduce the gay partner and the new baby? Or is this simply a party at which the visiters will be in attendance?
If it is the former case (which is what I think you are suggesting) then I think you need to decide what sort of message your attendance or lack thereof makes to the family. If it is the second case, then you are possibly making too big a deal about it.
Would attending the party imply approval of the gay couple? If so, to whom? Regardless of whether or not you attend, you are going to have to explain your position to your teenagers. If you do not attend the party you need to accept any consequences. Will avoiding the party be just to benefit you and your immediate family or are you hoping to send a message?
I think it would be morally acceptable for you and your wife to attend without your teenagers. (Chances are, they aren’t that especially interested in this party anyways.) If your mother-in-law is putting pressure on you, just repeat that they have other plans. Be polite but make it clear you neither owe her nor will offer her any further defense of your decision.
Alternatively, I would arrange for you to take your children and the children of any other family members to a movie, miniature golfing, water park, or whatever, and just have your wife attend the party. (To be polite you may need to say hello to the cousin and make some simple acknowledgement of the partner and the baby. “You are looking well.” “Cute baby outfit.”) Everyone will know that the real reason you are not at the party is that you do not approve of the situation but you are doing it in a charitable fashion.
Is the main point of this family party to welcome/introduce the gay partner and the new baby? Or is this simply a party at which the visiters will be in attendance?
If it is the former case (which is what I think you are suggesting) then I think you need to decide what sort of message your attendance or lack thereof makes to the family. If it is the second case, then you are possibly making too big a deal about it.
Would attending the party imply approval of the gay couple? If so, to whom? Regardless of whether or not you attend, you are going to have to explain your position to your teenagers. If you do not attend the party you need to accept any consequences. Will avoiding the party be just to benefit you and your immediate family or are you hoping to send a message?
I think it would be morally acceptable for you and your wife to attend without your teenagers. (Chances are, they aren’t that especially interested in this party anyways.) If your mother-in-law is putting pressure on you, just repeat that they have other plans. Be polite but make it clear you neither owe her nor will offer her any further defense of your decision.
Alternatively, I would arrange for you to take your children and the children of any other family members to a movie, miniature golfing, water park, or whatever, and just have your wife attend the party. (To be polite you may need to say hello to the cousin and make some simple acknowledgement of the partner and the baby. “You are looking well.” “Cute baby outfit.”) Everyone will know that the real reason you are not at the party is that you do not approve of the situation but you are doing it in a charitable fashion.