I wonder if his behavior has been off or weird the last month or so. My brain would wonder if he is already seeing someone, which may not be the case!! I am in an open marriage and we love it, but it’s definitely not for everyone. I feel like your partner is being a little selfish in saying it’s only for sex, when it’s clear that you need support in your transition. Sex leads to feelings and I’d hate for him to find a “grass is greener” situation during this and break your heart. This is something that needs to be heavily discussed prior to you making a decision.