This will force many same-sex couples to choose between their job and their relationship. The reality is that same sex-marriage is available only in a minority of countries, making it inaccessible for most couples. Also, a diplomat may not want to "out" themselves and their relationship to their employer and country of citizenship. It is a step backward from equality.
"US halting visas for same-sex partners of diplomats" - CNN
https://www.cnn.com/2018/10/02/politics/same-sex-couples-diplomatic-visas/index.html