Welcome to the group. I too joined an year ago and started wearing thongs 18 months ago. It was a tough transition time for me as well (more because of difficulty in finding the right thong). Regarding your thoughts on men thongs being a feminine clothing, i believe this is too far from truth. Thongs couldn't be more masculine. 2 reasons from my side:
Built/construction of thong: It has a pouch! I used to wear briefs, and to me they were feminine (with no space in the front for jewels). I tried boxer briefs as well and they too looked as if they were made for women. I am not too big there, but none of the conventional underwear felt masculine to me (becuase of anatomy).
People's Perception: Now, this is where it gets tricky. To me, there's nothing feminine about it. It is an underwear made for men that's comfortable. In my social life, only my gf knows that I wear thongs (in fact she got me into wearing thongs). She finds it too sexy (in a masculine way) on me. It accentuates the front and it's a turn on for her. Her opinion is that underwear should be small, comfortable, functional and appealing. Thongs tick all the boxes. But, when I read public forums such as Quora, I found that most people perceive it as a feminine clothing. Now, it's up to you how you take this. If your partner thinks it's feminine, talk to them and open their mind towards thongs. It's very likely that they too are brain washed by comedies and spoofs showing men thongs in bad rep.