I've read one! I forgot the title but its prequel is titled Jealousy.
You probably mean Yondaime Ooyamato Tatsuyuki :)
I was talking about http://www.mangago.me/read-manga/yondaime_ooyamato_tatsuyuki/ but no rival seduced there. He used threats instead... But in its prequel http://www.mangago.me/read-manga/jealousy_scarlet_beriko/, there's seduction but it is not any seme who used seduction. It's the uke. (^-^)' v Peace!
Kedamono Nanbiki?
is there any yaoi manga that the seme's rival manages to seduce the uke and do it with him? i don't care whether the uke ends up being with the rival or not, or there is drama lol.
Sorry for a weird request, I recently a manga that has a rival but was actually a tease Σ( ￣□￣||)