Hi there, wow I just randomly saw your question and I feel your quandry but in my mind, speaking as a woman who's been cheated on by a few guys, this is a BIG RED FLAG! If he's on Zoosk or any other site similar to this, you shouldn't brush this off. Zoosk is a dating site, plain and simple. You already know this but your not trusting your own instincts. Denial is a place I've been in, believe me and it only hurts you. The fact that you are pregnant doesn't help. It's also going to hurt your child if this goes on any further.
You know what's right for you, sticking around with a cheater who needs constant attention from other women is NOT what you will want when you have a small child around.
Think about it!
Peace, I hope you find some answers soon and I wish you well.