There is a build-in morph in morph/genital called something like "extreme expansion". This will spread the vagina very wide. In morh/hip there is a build-in morph called something with "pubic area wide", you can try this too, for instance. Every morph that widens the pubic area or hip/pelvis, should add some width to the vagina, in theory. You can use the plugin Auto Labia to spread it wide when opening the legs, too.
I say "in theory" because this body area is prone to multiple issues. I think the critical point might be the colliders, not bones. There is a somewhat complicated plugin called Collider Editor to edit them.
Edit: OK I just overlooked the word "guy", sorry. There are some anus morphs around to open the butthole wide, etc. But I am no expert at this. I don't know if the Collider Editor is working well with male figures (as those are completely different meshes and it even misses some of the lower body colliders). Though, the male figure is somewhat buggy from the start and the anus is not working very well in general.