Think of this as one of many talks about this issue over the years. Plan that you will need to gradually disclose to your daughter what she’ll eventually need to appreciate, and expect that over the years the topic will need to be re-visited no matter how well it is explained on earlier attempts. Don’t put too much pressure on yourself to get it all right the first time. If you explain it poorly or incompletely, you can amend how you choose to explain it at a future date. This is not something that will be solved in one perfect visitation of wise words.
Besides that, consider that your ex-husband now expects to be treated just the same as if his new lover were female. Would you expect yourself to pretend it was OK for him to leave you for another woman? No. Would you expect him to portray his actions as totally OK, if not the inevitable result of “who he is and how life worked out”? Let’s face it, that is the way to bet. You can see that not wanting to make a demon out of him is miles away from wanting to pretend he’s not done anything wrong. He’s doing what he’s doing, it is not right, but he thinks it is. The best you can do is to treat him as if he had a case of invincible ignorance. (From the prospective of the Judgement Seat, maybe he does; that is not ours to say.)
OK, that is the court you have to play on–that is, on the court of common concepts of what are good manners and what is good parenting–so start there. Your daughter is not comfortable watching her father kiss someone other than her mother. You can talk to him about the issue in those terms: that is, that it would be better for her if he not be open about his affection to someone else, especially for the time being, just as you expect your daughter is not going to want to see you kissing some other man. Tell him you think you both ought to respect her feelings about this. After all, many adults do not like to witness anybody’s public displays of affection…and when she has her own future partner all over her at some future Thanksgiving, you’ll be on stable ground to tell her to save it for another place other than Grandma’s house.
The day is going to come when you’re going to have to broach the subject that what your husband is doing is objectively wrong. You’re also going to need to teach your daughter about the very real difficulty faced by those who have intense romantic attractions for persons with whom they may not morally have a sexual relationship. It is not about making people out to be bad people; rather, it is about the very real moral boundaries that even very good people have to live within, and having compassion about the temptations faced by others and how they handle them.
But one day at a time. You ought not tell your daughter you did so, but do tell your husband you are giving him the chance to show his daughter’s feelings respect on his own initiative. If your daughter reports that he is doing it again, then you’ll have no choice but to teach her how to gently and yet firmly advocate for her own feelings and her own personal boundaries, even to people who choose to ignore her requests.