He is trying to justify his own cheating desires by putting it on the backs of all men saying its normal we all do it, not all men cheat or think about cheating, lots of people are actually very faithful.
So you can shut him down on that thought right off, it is a tool to make you think it's allowable for him to do, or want. If he needs more and doesn't think 1 partner is enough then he shouldn't be in a monogamous relationship.
He is hurting you by telling you that, so what does he think blurting that out won't hurt you, he doesn't want to hide it because after 9 years together you probably have lots of in common friends and people who know he is your partner and might see him out when and if he does cheat and someone will eventually let you know, he probably knows this fact. So is why he isn't hiding what he clearly wants and plans to do.
No, he is making it seem like he wants you included by the threesome is only a pass for him accepted by you, it isn't for you and him in his mind, even though you'll be there too.
If cheating is unacceptable to you, then say so, and if he cannot respect your 9 year relationship then maybe he should leave and have all the sex he wants with others while he is single.
Don't let him try to convince you it is good for you, or he is thinking of you too with this, he isn't.
No, don't believe him, he's worked this out to play you into agreeing to him cheating and I bet then he goes and brags to all his mates how he played you and he gets to cheat on you and you allow it.
If it was a BF of mine, 9 years in and told me all that I would leave because I know he is going to cheat regardless. Wish you well.
It's not what you have, it's what you do with what you have that matters.