Because he wants her to envisage him naked, it's sexual. Would you say that to a male friend of yours over coffee?
If it's not sexual - yet. It certainly is emotional that's why they have engaged for so long. This isn't a couple of flirty texts is it. He obviously fancies her too op, he likes her, he likes the attention she is giving him. Don't fall in to the trap of thinking this woman is chasing him. He sent a bloody pic of him and his son - that's actually very personal. That's moving away from sexual/flirty banter to 'I'm going to share aspects of my life with you'
I'm not saying LTB this instant, this could be salvaged I suppose BUT it's your Dh that has to do the saving. You know it's more than what he is saying, he is trying to fool you - even that is a basic lack of respect for you, he clearly thinks your stupid enough to fall for it.
He should :
Put his hands up and admit what he has done/been doing.
He should give you full access to his phone so you can go through it all
He should be asking for your forgiveness for crossing the line
He should be willing to do what ever you ask to start building trust back
He should be genuinely sorry and I mean heart felt - in which you believe him.
op don't turn a blind eye to this in the name of a quiet life or keeping family on an even track. My exSIL had 20 years of this from BIL and in the end mil, FIL and BIL started blaming her for being 'misrable'. She devorced him last year.
Make you stand now, otherwise this will not be the last time x