There are lots of sins, and ALL of the separate us from God. In fact, the sin that Jesus himself most spoke about was our treatment of others and not judging. Good luck finding a church without any sinners.
Another thing to think about: why during mass are you preoccupied with this pair of people? Did you see them actually receive communion, and if so, why were you watching them instead of focusing on Jesus in the sacrament?
Obviously, this is a tough situation for you. Maybe pray that God will help you see them as the people that He created them to be. Ask Him to remove judgement from your heart and make you more fully present during mass. Pray for them too while you’re at it, and pray that they will find grace in dealing with whatever struggle they may have. It’s hard to judge someone when you’re praying for them.
From Luke Chapter 6: Judging Others.*
37
x “Stop judging and you will not be judged. Stop condemning and you will not be condemned. Forgive and you will be forgiven.y
38
Give and gifts will be given to you; a good measure, packed together, shaken down, and overflowing, will be poured into your lap. For the measure with which you measure will in return be measured out to you.”z
39
And he told them a parable, “Can a blind person guide a blind person? Will not both fall into a pit?a
40
No disciple is superior to the teacher; but when fully trained, every disciple will be like his teacher.b
41
Why do you notice the splinter in your brother’s eye, but do not perceive the wooden beam in your own?
42
How can you say to your brother, ‘Brother, let me remove that splinter in your eye,’ when you do not even notice the wooden beam in your own eye? You hypocrite! Remove the wooden beam from your eye first; then you will see clearly to remove the splinter in your brother’s eye.