While I don't think she's their child substitute, I agree it might be the opposite. That is, they probably aren't "getting" her because they don't have children and want something to "replace that", but on the other hand didn't have children so they could save that money for things they actually wanted, such as a girlfriend. That much makes sense to me, and I know a lot of CF people who don't have children so they can travel more, or things like that.
Me: 31F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon