I speculation, GML (but not spoilers!)
The scenario of gay man fathering baby with friend who has no suitable man in her life has already been done, and it would be excessively repetitive to have it again (especially as the plotting/writing of late has been below the previous standard, so the whole thing would look like a pale imitation).
Sean isn't ready to adopt (refused to embark on assessment process), and I can't see Maria consenting to be a surrogate.
So the options seem to be that Marcus leaves Sean for Maria, agreeing to settle for a best friends plus fatherhood arrangement. Or this is paving the way for Liam to be "inherited" by Marcus during some absence by Maria.
Talking of gay men in Weatherfield, btw, where is Mr Gay Weatherfield himself? Presuambly he's still living in the flat, as his mother's is so crowded. But we haven't seen his face at all recently and I've lost track of what he's meant to be doing. (Ditto Owen, Anna and Katie).