Why does being LGBTQ+ always have a stigma with being unfaithful in relationships?

It's because of social stereotypes that regard LGBT people as overattached to sex, and so more likely to look for it anywhere, even if they are in a relationship. Like all stereotypes, you can fight it with communication, by spreading knowledge of what it really means to be LGBT.

Going a little outside the main topic, I have always found that people like listening/hearing/pay attention to the bad news more than the good. Going more specifically, it is a lot more common to find individuals in the LGBTQ+ community cheating on their significant others. It being more common, not being as many LGBTQ+ people as the heterosexual community, and so on.. really leads to this negative stigma in our community. Yet, I don't believe it is in the majority. More faithfulness and purity stigmas should be shown more in the media! Also, I know a lot of gay men in relationships that like to include others in their sexual activities. Some would look at that as unfaithful. I, however, do not.