Do you think either Eric or Donna have had sex with someone else?
Hi anon! Thanks for this ask.
Eric is the more simple side of the equation, so I'll start with him.
Although he was characterized as a typical, horny teenage boy throughout the show's run - and that included looking at porn and admitting to attraction to and fantasizing about other girls (like Miss October, that poster he has in his room of Farrah Fawcett, the Playboy magazines he enjoys, the X-rated film he saw with the guys, etc) - there was never any indication that Eric had sex or even got close to having sex with anyone else during the show. I firmly believe this is because he was so head-over-heels for Donna.
Now, Eric did date other women. He did have plenty of other women (and one guy!) who were interested in him. He did kiss and make-out with other women.
Kate (S1xE9 "Thanksgiving")
Buddy (S1xE11 "Eric's Buddy")
Shelley (S3xE15 "Donna's Panties")
Katie (S4xE4 "Hyde Gets The Girl")
Connie (S4xE7 "Uncomfortable Ball Stuff")
Emily (S4xE16 "Donna Dates A Kelso")
Leslie Cannon (S4xE22 "Eric's Corvette Caper")
Chloe the Slurpee Girl (S6xE20 "Squeezebox")
So he had the opportunity to do it if he wanted to. But he tells Donna in the season 6 episode "Squeezebox" that he didn't have sex with the Slurpee Girl and that he didn't even tell Donna about her because she didn't matter/wasn't serious to him. The whole time he and Donna were broken up, he never stopped loving her.
This tells me that Eric's never had sex with anyone else, because even when he and Donna are broken up she has an emotional hold over him. There are also indications that sex is meaningful to Eric, like it seems to be to Donna. Not something he views as casual, or something he would do with anyone. At the end of S4xE25 "Eric's False Alarm", he tells her that when he found out she was going to have sex with Casey, it felt like the day after they'd broken up all over again. It was heartbreaking for him, it refreshed his pain to think of her being that way with someone else. In season 5 when he and Hyde are sarcastically describing how sex works to Red after Kitty thinks she's pregnant, Eric starts out, "When a boy loves a girl - " (Hyde interjects to say he doesn't have to love her). Etc.
Did Donna have sex with Casey Kelso? I don't think so. Here's why.
In S4xE25 "Eric's False Alarm", she comes to the basement the morning after she and Casey had a motel room and Eric asks her if she and Casey did anything. She says no, they didn't have the chance. They spent all night in the parking lot with Leo and his hootie mamas, because someone kept pulling the fire alarm all night (it was Hyde). Now, could she and Casey have had sex between this episode and the season 4 finale, when he dumped her? Sure. It's technically possible.
But given how explicit this show tended to be about when characters had sex - I doubt it. It also makes sense that two episodes later, Casey is dumping her because he doesn't think she's worth the hassle - aka, she hasn't put out yet.
My headcanon is that Donna and Casey messed around/had a physical relationship, but no, they never had sex. After her talk with Eric in the basement at the end of "Eric's False Alarm", Donna reflects and realizes she's not ready to have sex with Casey yet.
I also don't believe Donna slept with Randy.
There is one episode in season 8 (and I am not going to bother to look up the episode and exact dialogue, because it's season 8 and who cares amirite) where Donna, Fez, and Jackie have an exchange. Jackie asks Donna if she and Randy have had sex yet. Donna gets defensive and doesn't answer the question. Fez says he thinks it will happen soon, because lately Donna has started shaving her legs all the way up to her thighs (ew - creepy Fez alert).
Once again - I think Donna did things with Randy and got close to having sex with him, but I don't think she actually did.
Similar to how she got close to moving in with him. She was forcing something that wasn't meant to be, accepting his offer to move in together because of her situation (she needed a place to live after Bob moved to Florida) and not because their relationship was that serious/ready for that next step. After she moves her stuff in and Randy gives her the key, she seems to take in what she's done and freaks out. She leaves, and guess where she runs to? The Forman's house. Eric.
I think Donna realized just in time that she was using Randy as a replacement for Eric - who she missed achingly. It wasn't fair to Randy and it wasn't going to make for a healthy relationship, so she ends it. Before they had sex - or moved in together.