While traditional baby showers are typically thrown for a hetero-couple that includes a pregnant woman, the situation for the same-sex couple you're planning for will likely be more unique. Because of this, Baby Shower Easy notes the importance of considering the circumstances of the specific couple in your life when organizing their shower.
It's possible that the same-sex couple has a third person in their life, such as a surrogate carrying the child, who they would welcome at the shower. Simple Surrogacy explains that some couples might want to invite the family of their surrogate to the event or take the opportunity to give them gifts, as well.
Some couples choose to use a known sperm donor to become pregnant, per Co-Parent Match. Depending on the agreement between the couple and donor, the man might be acknowledged as the child's father or serve as a third co-parent, in which case the couple might want him to be present at the shower.
It's important to clarify the wants of the prospective parents here, as some might not want to include or discuss the third party involved in their pregnancy at all. Establishing boundaries is a good idea, and will help you give the couple the baby shower they desire.
