-Opposites attract
-Ryou might make Marik less evil
-Marik could wrap Ryou is his cape
Reasons against:
-Ryou is too angelic for Marik
-They wouldn't look cute together
-Marik could wrap anyone in his cape
-Marik's hair is too hedgehog like.
-Ryou likes kitties
-Ryou needs someone sweeter than Marik
-Ryou's sane
-Marik would be too dominerring for Ryou
-Ryou is in love with someone else
-Marik is in love with someone else
-Bakura wouldn't like it
-Malik wouldn't like it
-Bakura would probably get homicidal towards Marik
-Ryou would be too scared to be alone with Marik
-Ryou has his own yami
-They've never really been shown together
So as this couple is so obviously wrong:
MarikxRyou