I have been with my partner 8 years and we live together with a mortgage. During this time our relationship has been pretty uneventful. We both have friendships of the opposite sex, no issues there.
However, recently a colleague at my partner's work invited him to go cycling with her. So they went cycling for 6 hours on a Saturday and then started going after a work roughly once or twice a week. My partner stated he hardly knew this woman before they went cycling as they are in different departments and she only works two days a week. So they were colleagues who just said hello in passing.
My partner was then surprised by a text he had received from her after he cancelled cycling and showed me and I felt it was mildly flirty and inappropriate. I then saw previous texts where she had text him out of the blue mundane things about what she had been up to and trying to keep conversations with him going. She would also invite him into her house after cycling for cups of tea.
The upshot is, I no longer feel comfortable with him going cycling with her. To me her texts are flirty, so I feel disrespected. My partner agreed one was a bit strange but he is confident she is not interested in him like that and doesn't know what possessed her to send that text. He wants to continue cycling and says if I stop him he will resent me. The issue is we are constantly arguing every time he goes cycling. We have never argued like this before and it's like living in hell.
I feel like my boundary is that he should cut contact with people who appear to be pursuing him romantically, not spend increasing time with them 1:1. I think it's better to nip it in the bud now rather than carry on possibly leading her up the garden path. I suggested he join a cycling group (she is part of one) but he said no he likes the flexibility of going whenever he wants not just when a group is going.
He feels his boundary is that I should trust him and not control/dictate what he can/can't do. Which I understand is completely fair. But now we are stuck at loggerheads unable to resolve it.