OP, you already have a lot of people speaking sense here, but I think @Techway has really hit the nail on the head in saying that he had to take steps to sex-chat with a stranger. He didn't fall ass-backwards into an accidental sex chat with a random, he sought it out.
He did that knowingly. Short-term-Covid boredom is so weak an excuse it doesn't even merit discussion, but if (and I think this is a big if) the truth is that the lack of sex and attention over the last 10 years has been so chronic that he feels he can only get validation elsewhere, that will not have happened overnight. (Sidenote: does he make you feel sexy/desired/appreciated?)
Over the course of a 10 year relationship he will have had 3650 days (give or take) to TALK TO YOU about this fissure opening up. He could have told you he felt distant or unwanted, he could have suggested some intimate time, a massage, counselling, sexy dice, WHATEVER.
If the lack of sex and attention was bad enough to end the relationship, he might also have done that.
However, I doubt he even gave it that much thought. I think your relationship was probably ticking over just fine and when he went looking for an illicit thrill, he probably didn't even consider why he was doing it and what he might mose. He probably thought "can I get away with this?" and dashed off a dick pic.
He showed you no thought, consideration or respect (all things that all of us deserve from our partners) and tried to make his deliberate actions your fault. Try to get as much space as you can, talk to a friend/sister/colleague you trust in real life and start thinking about what you need. xx