How do I address my partner's cuckold fantasy without pressure? I want it as a fantasy, not as a reality.

[ThatAquaRobin]

I've been with my partner for 8 months and generally sex is getting better and better.
He likes a bit of kink, smacking, me hitting him with a belt, that type of thing. Also rimming, a little bit of anal play, just fingers at the moment.
One of the things I love about him is how he is so straight and respectable, but can switch to absolute filth when in bed
BUT there is one thing I don't know how to address. Sometimes he will whisper things to me during sex about cuckold fantasies. Like him wanting to watch me with another man,while he gets off on it.
Now while I'm fine with the fantasy and with the dirty talk, and also would watch this type of thing on screen. I absolutely don't want to have sex with another man.
I have said to him early on that I only want him, so he knows this, but again last night he whispered the same things during sex. To be fair there sex was great.
I guess I just want hints on how I should again emphasise with him that this is 'just a fantasy' right?
I sort of want the reassurance that he's happy with our sex life 'as is' and that I'm not suddenly going to get dumped for someone who will do all this.
In other respects all is good. We've met each others kids, been on holiday loads, that type of thing.
I'm.50. He is 53. Both were in long marriages before that ended.

Be Blunt . Tell him in a very clear moment about what you are thinking .
And you are invested in this relationship as is he .
Ask if this is a deal breaker for him ? This is
in person conversation and do it while casually having a meal so there is no excuse to get up and leave in the moment unless he pitches a fit .
Open conversations like this will set the tone

Thankyou @Winterbolts
Anyone else?

Yeah you need to be up front with him and tell him that although you might be ok with exploring the hypothetical fantasy, that you would never consider this in real life.

Perhaps you could say that you like knowing what filthy thoughts he has during sex and that you would be more comfortable leaning into the dirty talk of his cuckold fantasy, if you felt reassured that he wasn't ever going to push for it to become a reality because that would be a hard limit for you.

This is a new relationship. I would be very concerned about him wanting to push boundaries and trying to coerce you over time to have sex with another man. If that’s the case, it’s easier to walk away now, than 2 years down the line.

I've got no concerns about him pushing boundaries. But you're right I do need to clarify things. He's the gentlest kindest man I've ever dated, but just has this naughty side to him. He's always asked at every stage before we try something. And to be fair it's very light kink. Whenever I've asked before, he tends to get embarrassed when I bring it up when we're not 'in the throes' so to speak

I would video chat with someone and let him watch from behind the laptop. You can decide what you show, what you do and what you talk about and he can see you engaging with another man in a sexy way.

Be firm that it will remain his fantasy. It could be you give him an inch and he’ll take a mile.