Responds to Becky G Cheating Claims

Soccer star Sebastian Lletget says he's the victim of an "extortion plot" amid rumors he cheated on his fiancee, Becky G ... but admits he did, in fact, screw up.

It started last week when an Instagram user claimed the FC Dallas midfielder cheated on the award-winning singer with a woman last year while at a nightclub in Madrid, Spain.

Along with the accusation, the person also seemingly provided receipts including alleged video, text messages, DMs and audio from Lletget.

FYI, Sebastian proposed to Becky G just 3 months ago.

After the posts went viral, the accuser's IG account was deleted ... but the damage had already been done. Days later, Lletget has decided to address the rumors (though not…