How to ruin a boys birthday

If someone can tell me how a 2 1/2 year old seeing his dad kissing a woman that's not mommy is distressing. I'd appeciate someone explaining it to me. Before answering take into consideration that mommy and daddy haven't been together since he was 10 months old. This is what ruined Shawn's birthday party today. Irrational mom goes off on me about how I'm hurting her and hurting the kids and Shawn just wants to stay with daddy but no, it's time to go, pack up everything and we're outta here. I could care less if Jennie gets mad at me but this had nothing to do with Shawn or Donnie and it was just flat out wrong for her to ruin his birthday because of it. If she ever wondered why I'd never ever consider getting back together with her, today was another example to put in the books.There are some that might say told you so though I dont really expect that to happen. I just need time to let this wash out of my system and then the waiting game is on again. No doubt, sleeping with her in february and then announcing a few days later that I have a new girlfriend could have been hard on her. It doesn't say you're a cheap whore and all the other things she accused me of treating her like today. It just means we made a mistake, yes we cause like my good friend Syren said, it takes two to tango. There will be some that will think I should despise her and never talk to her again but when this blows over and it will, she'll still be the mother of my children and that's how I'll treat her.As for today (well it's techincally the next day now) I'm pissed and sad and just dont want to talk to anyone right now. I dont know what to say. Luckily I caught the few friends that were supposed to come over before they left the house so they didn't have to waste a trip. Today just sucked all around.