Cheating is never right. It will wreck your marriage. And that child you have together? You cheat, and you're going to lose him. Yeah, sure if you hire a good lawyer and can prove you're a good father, you may get joint legal custody, but it's going to amount to every other weekend and maybe one weeknight. Oh yeah, and half your assets. If she's been staying at home to raise that child, then expect to give her more than half of the "community property" when she leaves you for betraying her trust. I know what I'm talking about because leave with the kids is exactly what I did when I found out my husband was cheating. "I'm not getting enough sex" is going to sound really lame when your wife tells all of your mutual friends what you did.
You can't show up after work and expect her to want to drop to her knees and suck your dick. She's tired. You're tired. You've got a kid. You want more sex? Help her out around the house. Hire a babysitter so you can go out on dates and then put some effort into it. Bring her flowers for no reason. Offer to take the kid for an afternoon and tell her to go shopping for something for herself. Kiss her, often. Tell her that you love her, that she's beautiful, that you want her. In other words: put some effort into being a good husband because in real life, most of us are not like the women in porn who are horny all the time and constantly want endless marathons of kinky sex.