First of all, I feel I can help here because my ex was one of those men who needed attention from other women and couldn't cope with the idea of monogamy even though he hid everything he did. He only ever sexted to my knowledge but it probably went further although I'll never truly know.
First thing:
Aside from the fear of being alone/breaking up your marriage how do you actually feel about this? Do you think it's acceptable or ok? I'm guessing you don't.
Please do not ignore your feelings on the situation and what your own stanards are. I ignored it and tried to be ok with it and was looking over my shoulder for the rest of the relationship. Every time his phone beeped I was trying to see who it was. It caused so much anxiety it was unreal. I should have just left and I regret it so much but I stayed for another 4 years and even got engaged and bought a house with him.
Ultimately, if you're ok with it and it's something you are willing to accept, is there any point in bringing it up with him at all? Probably not.
When people do things like this and hide it, they hide it because they know it's wrong. They hide it because they know they're liars and they know they're doing the wrong thing and that it would cause upset in their life, so they cover it up and hope nobody finds out. The truth always comes out though as you're discovering.
It's likely that when you confront him with this stuff he'll deny it, brush it off, minimise it and turn it back on you and you need to be prepared for that because that is classic tactics for someone who can't accept responsibility. Are you prepared for that to happen? Because it will. They don't like being called out on their bullshit so they will gaslight the shit out of you and make you feel like you're blowing stuff out of proportion. The one thing I'd say to you is: Hold onto your standards no matter what, and hold onto the evidence no matter what.
Are you willing to end your marriage over this? What lengths are you willing to go to to ensure this doesn't happen to you again, because ultimately he's doing this to you so that he can have his cake and eat it. So you need to consider what steps you're going to take to enact consequences.
For me personally, I shouted and cried but there were no consequences, I never left, I didn't take anything away from him, I just forgave him and tried to be ok with it and internalised all my anxiety about it and my worry. Whenever I would talk to him it never did anything because firstly he'd often shout at me and gaslight me, and secondly he's already shown me what he's capable of so his words of reassurance when he did give them meant absolutely nothing.
Consider all of this. Take photographs of the evidence and consider what you're going to actually do about this.
If you decide that you know what, I'm not accepting this and I'm divorcing him, then see a lawyer and get your ducks in a row first so that he doesn't see it coming, because men like this are incredibly persuasive, charming and if they're not those things they're abusive and crazy making all in the effort to maintain the status quo they have set up for themselves.
Just remember there is nothing he is ever going to be able to say that is going to reassure you that he won't do it again, and unless he chooses of his own free will to go to therapy and find out why he did this and do some serious soul searching, the flowers and the trips away are going to be of no use whatsoever. He has to choose to take steps to fix the marriage and his own flaws if he ever wants to restore trust...and FYI, don't do what I did and make it YOUR job to be ok with it/be trusting - it isn't your job, it's his.
I'd say there is enough evidence here to suggest he's incredibly dishonest, I think sending gifts to other women is completely unacceptable if they aren't mutual friends and remember that hiding it from you says everything. He's promised you he won't do it again once, he lied and now you're left to decide whether you want to give a second chance. I would strongly suggest you don't. Don't be like me. Don't waste your life on people who don't deserve it.