"Assuming that those same-sex unions can provide good role models to the child (-ren) for both sexes."
Again, this is irrelevant. Explain to me how denying legitimacy to SameSex Marriages magically teaches good (and one can argue that definition forever) Male and Female role models to children in same-sex households. It does not. All it does is hurt those children by disadvantaging their caregiving parents and their household.