Sub-Zero,
I only had to screw up and not have a condom 1 time and never forgot it again. I swear if I have to pin the wrapper to the inside of my leg I'll have it always & forever. That really sucked, but you know that. You can't change the past!
You've been around the forum for a while and you pretty much know your options. To say it didn't hurt when you messaged Marsha that pic of Beth would be a lie. I wouldn't say you're totally f'd on that one but it's going to hurt for a while. Women don't tend to forget that stuff like men do. You're probably going to have to wait a respectable amount of time before amping it up with Marsha again.
As far as Beth goes she seems like the kind of girl you can control pretty well. Maybe you can slot her as the friend with benefits possibly instead of BF. I think you can heat up the attraction and turn it that way. Bear in mind she's probably trying to put you in the zone because she wants more "comfort" before more sex. Still I get the impression she'd fold to sexual pressure again without more comfort.
Try to slot Beth as a friend with benefits so you have what you want for a while. Go back to being friends with Marsha, keep her in your life, don't try to escalate things for a while. Basically, until she is convinced you're a good guy and not a player, that sort of thing. Then try again!

either it'll work or it won't.
Good Luck Sub!
Jon