When someone comes to RCIA with their girlfriend/boyfriend, maybe romantic partner, fiancee, spouse, or even a sibling or other family member, that is wonderful. The relationship might, however, make the earnest discernment have difficulties. A catechumen might feel either overt or even subconscious pressure because of that close emotional relationship.
A sponsor ought be someone where the catechumen can ask difficult questions, talk about their struggle, without the pressure to continue toward baptism because they do not want to hurt the person they love.
What we do is encourage the loved one to be part of the RCIA process, to sponsor someone else in the group of catechumens who may not know any Catholics to fill that role. This way they attend preparation together. We also encourage people to have a sponsor of the same gender, women with women, men for men, because when discussing intimate moral issues, there can be some awkward or even immodesty talking to someone of the opposite gender.
Sometimes the romance ends, the marriage breaks up, and then the convert has a spiritual relationship that is also severed.
Again, this is not law, but, comes from much experience with the catechumenate.