LA Galaxy defender Robbie Rogers claimed he was called "queer" several times by an opposition player as he made his comeback from injury.
Rogers, 29, had been out of action since May after undergoing surgery on his ankle.
He made his return in Galaxy's reserve side, LA Galaxy II, as they fell 2-0 to local rivals OC Blues FC on Saturday.
However, Rogers claimed he was the subject of homophobic abuse on a football pitch for the first time since coming out four years ago.
"In the heat of the last fifteen minutes of the game a player from the opposing team called me a "queer" repeatedly. To be honest my initial reaction was one of shock," Rogers wrote via Facebook.
"This is my fourth season back in the MLS and I've yet to hear another player use that or any other gay slur during a game.
"I quickly became enraged, I spent the drive home wishing I had channelled my inner [Zinedine] Zidane and punched or head-butted this player even though I knew punching this person wouldn't have helped either of us, my team, or the greater cause of advancing equality in sports.
"I went to bed upset last night. Angry at this player and his ignorance. Angry at myself for not doing more in the moment. Sad [that] we still live in a time where this kind of intolerance still exists in my sport and elsewhere."
He added: "This morning even most of the anger has faded and the predominant emotions left are just love and gratitude. I'm happy that I practiced restraint worthy of my sport. I'm thankful for the many players on my team and even the opposing one who apologised to me for one man's actions.
"Today, I woke up grateful to work in organisation filled with so many players and coaches who have worked hard to practice tolerance of everyone and to help change a culture.
"But mostly I'm proud of myself. I am proud more than ever that I had the courage to come out as a queer man. I feel so fortunate to have gotten to share my story with others and to have gotten to play this sport I love so much as an openly gay person.
"I am, more than ever, thankful to have team-mates and a family that love and support me for the son, brother, partner, father and queer player that I am."
The USL released a statement on the incident, saying: "The USL has begun a full investigation into allegations of homophobic slurs directed at a player during last night's Orange County Blues FC vs. LA Galaxy II game.
"The USL has zero tolerance for any discriminatory behaviour."