Doug did have an affair and this was addressed in the 2nd season about halfway through. The person who Doug had an affair with comes to town in her one person play and eventually Kate asks Doug to invite her to dinner.

That's my signature line. Sorry I forgot to put a space.
See below- there it is again!
It's from the movie "Sharkey's Machine".
And you wanna know the worst part? You're from outta state!

And you wanna know the worst part? You're from outta state!
HUH?

Yes, Doug had an affair at some point in his and Kate's marriage, but we never know really at what point. It's brought up a few times, especially in the episode when Doug covers for a friend of his that's having an affair. Kate tears into him about how guys are in a "club" to cover for each other. Doug comes close to having another affair with Linda Lavin's character, but wises up at the last minute.
Buddy's virginity was intact as far as we know when the series ended and she goes off to college. Zach (Leif Garret) tries to pressure her, threatening to hook up with another girl because "she's willing to give me something you're not". She says she's not ready and ultimately Zach stays with her anyway.
And you wanna know the worst part? You're from outta state!