My ex just started reaching out to me again, but in a seriously sexual way. Which I'd be totally okay with, if she didn't have a boyfriend.

I'd wait until i had solid evidence of her talking to you, then show her new boyfriend.

Thats the thing, as in shes "reaching out," I mean sending shit. I didn't ask, they were just sent. Shes also been coming clean about stuff about her and how she had a dream about her and I the other day

I'm not sure I'm understanding why this is a conundrum for you, sorry.
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It seems fairly straightforward to me. She's your ex, there's obviously a reason for that. But even if there isn't, you stated two things that should be instant red flags:
-she has a boyfriend, which you're aware of, and she clearly is aware you're aware of
-she's sending you this stuff out of the blue, unsolicited.
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The second issue indicates erratic behaviour and blatant negligence to me. Either she's bored or unhappy and sees you as an outlet for that.
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This ties into the first issue where she knows how very likely it is that her boyfriend could find out about this. Either that or she's an incredibly dense person, but I don't know her well enough to make that call.

Either way she's displaying a blatant disregard for his feelings, a blatant disregard for your feelings, and setting all of you up for a situation that in no way can end well. Even if her boyfriend never finds out, and she stays with him, then what? She lies to him for the rest of his life?
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Okay, that's fine, he's not you. But it displays a very ugly side of a personality that I'm not sure why you'd want to willingly engage with. If he DOES find out, there's now a significant chance he could decide to lash out at YOU because logic is rarely a factor when jealousy and betrayal are involved.
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The entire thing sounds like it's opening you up to a world of unnecessary drama that seems ENTIRELY unworthwhile just for a few booty calls.

I failed to see how this is serious, no offend ofc.
It's just very simple.
She wants to cheat on him with you, if you're cool with that, go for it.
If you're not then either you expose her or block her.

Dont do it. Shes with another dude. If she wontbdtop threaten to expose her

@the_grinch I feel like part of your comment needs some clarification

My bad just woke up. Wont stop**

Ahhh that makes much more sense!

Ex must be ex. It's simple.
Another question: why are you ready for these relationships?

I agree with @rosalinas. Bang her if you want to. Dude isn’t your problem.

I think if you're wondering, it means a part of you wants it. She wants a sexual thing to happen too, and (from what I understand) not a relationship, so you won't hurt your feeling in the process, if you're in it strictly for *it*. Now it all boils down to the boyfriend. You can 1. ask her if she's unhappy or what the matter is, and maybe she'll work on her relationship (but will very possibly be pissed at you), 2. wait until further development in her relationship (either they moving forward and getting more serious, or breaking up), 3. act based on your personal morals and opinions on the dude, provided that you don't seem to know him and probably won't, but it will hurt him if he finds out.
If the guy was an asshole, your choice would be simpler. But as you don't know him, there isn't a black or white answer. I wouldn't do it, because I wouldn't want to hurt anyone, but ultimately it's your choice. Sorry for the long and useless answer.

I've been there, I've done that and my advice is block her now this will not end well.

Totally agree with @xvarnah, it's drama that you can easily avoid. Ignore her. Block her, you don't need someone like her in your life. Imagine being her boyfriend and finding out your gf is texting some other guy sexually, that would hurt like a mofo. I know you didn't say anything about cheating, it was just brought up by other users but imo cheating is one of the lowest things you can do as a human being, you're an asshole if you do it and if you get hurt then honestly you deserve it. Anyways, my advice is just block her. She's out of your life, your relationship ended for a reason and if she's the type of person to be disloyal to her current bf then she's not a person you need in your life and she will be disloyal to you.

Honestly I'd block her at this point. I get that it isn't always easy to move on from an ex. But that fact is, there is probably good reason you broke up and it's more likely than not that reason(s) hasn't gone away.
Personally I don't see a problem with sleeping with any adult who is consenting. However I also believe it is important for all people involved in a sexual situation to be aware of the situation and able to make informed decisions based on that information. [ie. If you aren't exclusive, this should be known information, not necessarily who all you are sleeping with] So, I can't in good faith recommend sleeping with her when it is likely her boyfriend is unaware of that as a possibility. That being said, I also don't feel it's your responsibility to inform him.

Any update, @cotten_eyed_joe ?

Hey, sorry. I dont get on here very often anymore :/ but yeah, it stopped. I told her this whole thing probably isnt a great idea and we just ended it

@cotten_eyed_joe Yeah, I think a lot of us find life always seems to land us on here more or less often depending on other things.
I hope you've found some peace in your arrangement. I know it's not always easy to move past an ex.