If the people having these thoughts are really suicidal, then there really is a serious risk of them committing suicide, which makes what they're thinking not "just thoughts".
Also, while it may not be leprosy, one's thoughts and world view can seriously affect your partner. Partners committing suicide together does happen, and even when only one of them does it can be devastating for the other, and even if they don't being in a relationship with a suicidal partner can be heartbreaking.
I would seriously think twice, and three times even, before dating someone who was suicidal. IMO, suicidal people need to seriously get their lives together and overcome their issues with therapy before dating, or they risk ruining not only their own lives and that of their own family, but that of their partner and their family as well.
About 4% of the US population has suicidal thoughts (source CDC)
Its not leprosy, these are just thoughts. That will go away for most people. Or might come and go, same way as you might get sick at some points in your life.