it's your kids birthday, attend the party, you don't have to stay till the very end, consume some food & drink, conversate, have fun, be merry (don't be Mary!) and then leave.
I don't think you have to worry about you wanting her only for her looks & sexuality, seriously that's all she's given you a chance to want her for lately, what else has there been? This is a non-issue right now, don't sweat it.
