Respectfully, OP, I think you’ve had a few other threads recently about this Colleague (C) who was about to go on paternity leave, and if so, they are relevant here. Together they show a building of emotional investment.
If you are that poster, you’ve stated that you are lonely and need a friend. You’ve been traumatized by your H’s infidelity, and your job has been a lifeline. You also have a nice friendship with a female team member.
You’ve enjoyed chatting with C by email, at work, and the 2 times you’ve been to lunch with him. You described it as a lighthearted and easygoing friendship, with absolutely no flirting. However, your friends were concerned about your lunch conversation topics and also thought that C had overstepped by being too curious about your H and by asking questions about what he (H) would want in various scenarios.
Example: C mentioned that his Wife has weight issues. When you jokingly said you’d like to lose a bit, C asked, “Does your H have a problem with your weight? Also Odd: When you suggested ways that C could support his W during her pregnancy, he asked, “But what about men and how they’re feeling?”
When posters warned about developing inappropriate reliance or ego validation, you were adamant that your boundaries are strong and said that C had commented that you have a big personality and it was nice to chat without anything untoward going on.
Your H contacts escorts when you leave town (but claims to never meet them), but at the time of your first thread, you said you would never share your marriage issues with C.
By your 2nd thread, however, you had over-shared with C. On a day when you felt upset at work, he guided you to a private space where you told him about H’s infidelity. When he opined that men usually seek escorts to fulfill a need, you ranted that H can’t use that excuse, as you are the adventurous one. While C was surprised about it, he was a very sympathetic listener and he reassured you. You later greatly regretted confiding such personal information and hoped that the friendship was not ruined. When you messaged C to thank him and apologize for over-sharing, he replied that it was ok as long as it had helped you.
You are hurt that C is distant now. Perhaps his W saw the above message and felt uncomfortable. Perhaps C feels uncomfortable with the recent deeper dynamic and is stepping back. Perhaps he wants to pursue an illicit emotional connection, but private messaging while he is home is off-limits. Or maybe he is just totally focused on caring for his precious new baby and supporting his wonderful Wife.
If you are the above poster, I do think an emotional reliance has developed on your part. There’s been an escalation of investment, and that is a slippery slope. Step back now. Focus on making a decision about your marriage. The stress is eating you up. 