You'll often see me refer to "partner" in relation to my relationship status. My reasons may be different than most but I'll try to explain.
Like many TG I had a relationship that ended due to my being transgender ( amoung other reasons)which was monogomous for 15 years . A few months later I met a fabulous girl .She wasnt single though. She asked me at the time to not make her give up her other relationships. I never have.She was also 18 years my junior, and frankly I would not be able to keep someone in their sexual peak satisfied when I am in my 60's in 15 or so years She's my wife 7 years later. I've come to accept that she identifies as polyamorous, and embrace some of what this lifestyle has to offer. I have three "partners" , my spouse, a likely FTM, and a MTF. At different times, they have identified differently. For example I've been seeing my spouse, who has some FTM tendancies but wont transition as she is happy with her female self. The MTF partner has identified always at trans, but was VERY early when we met 5 years ago. The FTM partner, still identifies as female but will eventually transition FTM.
The key point here, is that their identification often changes over time. And if you have more than one partner it can become confusing, who's a he, who's a She who identifies as he but is still presenting as a she. So GF, wife, etc labels dont always apply 100% , and change over time. Partner is gender neutral, and covers all the bases, and leaves the gender issues out of it.
Seana