The emergence of AI companion applications has created novel forms of intimate human-AI relationships, yet empirical research on these communities remains limited. We present the first large-scale computational analysis of r/MyBoyfriendIsAI, Reddit's primary AI companion community (27,000+ members). Using exploratory qualitative analysis and quantitative analysis employing classifiers, we identify six primary conversation themes, with visual sharing of couple pictures and ChatGPT-specific discussions dominating the discourse of the most viewed posts. Through analyzing the top posts in the community, our findings reveal how community members' AI companionship emerges unintentionally through functional use rather than deliberate seeking, with users reporting therapeutic benefits led by reduced loneliness, always-available support, and mental health improvements. Our work covers primary concerns about human intimacy with AIs such as emotional dependency, reality dissociation, and grief from model updates. We observe users materializing relationships following traditional human-human relationship customs, such as wedding rings. Community dynamics indicate active resistance to stigmatization through advocacy and mutual validation. This work contributes an empirical understanding of AI companionship as an emerging sociotechnical phenomenon.