... Why else would she hide this from me except with the intention of cheating or at least wanting to leave her options in case the guy wants her.
The answer is that I just don't know why.
What are the options?
~ He's just a friend, who is staying over & there's nothing more to it.
~ It's not even true ~ no-one is staying there.
~ She fancies him and is planning on sleeping with him ~ a brief 'fling'.
~ She is in love with him & plans to stay with him.
~ She wants to have a relationship with both of you, but doesn't want to talk about the one to the other - because it's awkward.
~ It's a rumour to make you jealous & make you realise that you need to be honest with her.
Could any of therse be possible?'I'm okay with her wanting to be with other guys'
What does this mean?
Does she usually date other men, even though she is with you?
If you have said that you are all right with it, why are you getting stressed about it?
You are okay with her seeing other men, but you were thinking of spying on them at night or early in the morning?!
Does that really sound as if you are 'okay with her wanting to be with other guys'?
I don't think it does!
This is how it sounds to me.
You have said that she can see other men.
she is either seeing another man, because you have 'given her permission',
or pretending to see another man, to show you that you are not really 'okay with it',
or you are so concerned that you have said that you are okay about it, when you are not, that you have made a mountain out of a molehill.
It sounds to me as if you are not being truthful with her.
You talk about trust, but want to spy on her with another man.
You say that you are okay with her seeing other men, but you are finding this stressful.
If she is taking you at your word, that she can see another man, and is doing so, it's quite understandable that she wouldn't tell you. You may see it as trust; she might see it as rubbing your nose in it. By not telling you, she may be doing you a kindness ~ though of course she may not be doing anything at all.
Tell her the truth ~ that you are not at all comfortable about her seeing other men. She might be pleased. Most women would not want their partners to offer them this dubious freedom. Most want a committed 1:1 relationship.
'She says stuff like she's all mine'
'I'm serious about her'
Then act upon it ~ tell her you love her and want her and don't want to share her with other men, but want her in your life and your child's life on a permanent, serious & monogamous basis. (Provided that this is true, of course.)