I think she wanted Henry because she saw this reality as her second chance. If this is her happy ending, then she wants to have love in her life again.
Also, it’s some tasty icing on the revenge cake if Regina can have a child when the curse took Snow’s child away from her. Even if Snow doesn’t realise it, it’d still be enough for Regina to feel like she came out on top in that regard.
Sucks for her that the kid she ended up with can see who she really is, so it’s not working out quite as perfectly as she’d hoped.