i want to know

in Lgbt relationship ( any genders) does it happen that people cheat and other partner feels bad about it

How do you imagine that to be impossible?

Yes, LGBTQ people are like "straight" people in that they can be in a relationship and 'cheat'. And much like the rest of society, most people find this hurtful and it creates problems in the relationship/marriage.

Some people deal with this better than others, but this is the person, not the group they identify with (gay v straight or even partnered v married). This can even be a problem for couples (groups) in open relationships.

It boils down to trust and the feeling that trust has been abused or betrayed. The vast majority of people will be angry when their trust is betrayed.