I’m a mid-20s cis straight man. After my girlfriend and I finished college, she moved overseas to start her job. We’ve broken up twice and gotten back together twice. We are interested in opening up our relationship, but I have reservations. She wants the freedom to throw herself into her new world without the constraint of having to shut down non-platonic sparks. My girlfriend has brought up marriage several times. While she admits she doesn’t have a good track record with monogamy, she insists marriage will change that. Another concern: The last time she was in an open relationship, she cheated on her then-boyfriend with me. “No exes” was one of their rules, and I was her ex at the time. (I didn’t know she was with someone else.) Another wrinkle: When I confided in her recently that I had developed romantic feelings for another person, she asked me...