(Noun) the money spent on gifts or treats for one's significant other, in order to show devotion to said significant other.
Also may refer to the money spent in the efforts to begin a relationship with a prospective partner.
May be abbreviated to RT or R-Tax
1. "He'll suffer tonnes of relationship tax with a girl like that."
2. "She paid a lot of relationship tax buying him that car."
A price you pay for being in a relationship. If your partner wants to do something stupid and/or boring, often times you get suckered into participating for fear of being called not being a supportive partner or cold or a bitch or an asshole or something else lame.
Wearing that sweater your Mother-In-Law made that your wife thinks "adorable" but made that Leather Daddy downtown "give you the eye", you look like total fagbait in it.
My wife likes it when I wear it and this morning she said: "Why don't you ever wear that sweater my Mother made for you? She spent hours making it, she spent hours making it, you could at least wear it! Besides, you look adorable in it." I basically wasn't going to get laid tonight if I didn't wear it, it's the relationship tax.