I am not an expert on this and would prefer to see a reply from a member of the Transexual/Transgender community, but … ‘Sex changes’ do not happen overnight. Someone doesn’t just wake up in the morning and say ‘Oh Wait, I am a guy/girl.’ The fact of being transgender would exist before the relationship began and would therefore to a certain extent decide the qualities a person would seek in a partner. For example, someone born in a female bod who identifies as a straight male would probably want to date straight girls, so I think that in the example you give, both already have been aware of the situation and the issues relevant to their relationship. Because TG people also are required to live as the gender they really are and take hormones for years in advance of surgery, it is unlikely it could come as a huge surprise to their partner. As for whether they would stay together, that’s up to them; it’s the person inside you fall in love with, not the outside bits.