I never really thought about it till my boss who welded it for me pointed it out but I've never seen a dedicated trike that didn't have a step through frame aka girl's frames. I have seen a few bolt on trike kits installed on men's frames but still not often. I know it's easier to throw a leg over but structurally men's frames are stronger and look better.
Of course having a men’s-style frame will make it harder to get on and off. I think the big thing is you wanna be able to lock the brakes on the trike. Of course you don’t it want to just roll away.
BUT You don’t want it to move while you’re getting on and off. The bigger and heavier you make it the more important this will be.