男朋友

男朋友 is one romantic unit. Insert 的, and you split it into a male friend. Chinese speakers often say 男性朋友 or 男生朋友 for a platonic male friend. In everyday speech, 我的男的朋友 sounds clunky. The no-的 version 男朋友 is the one that means boyfriend. “我的男朋友打电话给我：“喂？” (My boyfriend called me: "Hello?") ── [《吃的东西和喝的东西》](/read/buying-food-drinks#s3)

If you say 我是男朋友, it means 'I am a boyfriend.' That only makes sense if the speaker is male. To say you have a boyfriend, Chinese uses 有: 我有男朋友. To say he is your boyfriend, use 是: 他是我男朋友.

一个 means one among a group. 我的一个男朋友 sounds like one of multiple boyfriends. For a normal relationship, say 我的男朋友 or 我男朋友. Use 一个 when you actually need to count or introduce him.