The actor played dastardly under-butler Thomas Barrow in all six series of the ITV show, which aired its last episode in 2015.
In an interview with the Radio Times, Rob claimed casting producers have found it difficult to see him as a “heterosexual love interest”.
He said: “I think audiences in the US can identify quite easily with the quintessential English gent and English lady, whereas a neurotic, dark, gay character like Thomas is a hard thing to put into the American market. It can lead to typecasting.
“Because Thomas Barrow is gay and is, essentially, the bad guy for much of the show, a lot of people within the industry can only see you as that.
“They might not be in a rush to see you as a heterosexual love interest. That takes time.”
He went on to reveal he had since turned down theatre roles as gay characters because he “really wanted to show something different”.
After the interview was picked up by various other news outlets, his comments caused a stir online, where he faced a backlash.
Some pointed out he had no problem with playing the role while he was being paid for it, while others pointed out how it shows how more LBGT+ actors should be awarded LGBT+ roles.
One of Rob’s biggest storylines on ‘Downton Abbey’ saw Thomas attempt to ‘cure’ his homosexuality.
The actor previously said that depicting the difficulties LGBT+ can face while coming to terms with their sexuality had led to him receiving messages from people in a similar position, saying it helped them.
At the time, he described it as “the greatest compliment of all”, adding to the International Business Times that they were “very humbling, poignant and heartfelt”.