HenSemi (TV)

Vulgar sex comedies with similar character designs. 

First of all - they're both about one's sexual life. They're also very strange and make me think "WTF is happening?". Both of them last for 12 minutes per episode and, finally, they're both from 2011 spring season. 

Dirty sex jokes in both anime, although Hen zemi is even dirtier and more lewd. If you really like sexual humor and want something more mature and with more well thought jokes, go watch Hen Zemi. Be aware that HZ has some poop jokes and other gross stuff that may not be suitable for everyone. 

Both of these are through and through perverse yet still retaining comedic content - and not just "haha pantyshots" level of comedy. 

Both have the "weird" seminar theme going on. The plots are really similar especially with the humor and both are great comedies, you'll understand if you've seen them both. 

Both animes are about a character joining a group, and learning about themselves in the process. Both are very funny, and deal with some of the more extreme aspects of society (extreme fetishes, extreme otaku(ism)) 

Both are about adult characters, and both are about a main character that finds a group of weirdo people. HenSemi is more sexual and deals with fetishes and such, while Arakawa is more about people that have delusions about themselves, like only being able to walk on a white line.