'wormhole' and 'selfcest' are the tags you want to be searching, though if you're specifically looking for it happing to guys then I don't think you'll find much
Here's something recent with futa; https://exhentai.org/g/3450978/64811153ce/
And something older with guy; https://exhentai.org/g/2704386/4f641c43f4/
Also shame on you for not titling thread 'thinking with portals'