The first time my girlfriend asked me to pay another man, she called it an investment in our relationship. When I discovered what she had promised him in return, I realized I might have agreed to something neither of us could take back. Natalie and I had been together for three years. We weren't struggling financially, but we weren't rich either. I ran a small business, she worked in marketing, and we were saving toward a house. Our sex life had always been good, although lately Natalie had been talking about exploring fantasies she felt she couldn't discuss with anyone else. One evening, she asked whether I'd consider an open relationship. I wanted to understand why before answering. She explained that she loved me but was curious about experiencing another man without ending what we had. She promised there would be no secrecy, and I agreed to discuss it rather than dismiss her. A month later, she met Sebastian at a private party hosted by a mutual friend. He was older than us, financially comfortable, and unusually attentive. He listened when Natalie spoke and remembered tiny details about her. When she told me he had invited her to dinner, I agreed she could go. She returned home happy. They had kissed, she admitted, and she wanted to see him again. I experienced jealousy, but I didn't accuse her of betraying me. We had discussed the possibility, and she had been honest. Then Sebastian made an offer.