I can't generalize it because I have only dated this guy and so I don't know about other geniuses!
He is a GEEK and is proud of being one. So late in the night instead of texting typical I love you... He will ask what is 2*3*3*547*571*....
and when you say that you dunno he will tell you that he has done a prime factorization of your mobile number at like 3 am in the night!
Also at midnight when most of the people in love talk about romantic and sexy stuff.. He has sung Bangladesh national anthem for me and talked about the great Rabindranath Tagore who has composed three national anthems!
He is very down to earth and considers me equally intelligent! (Which obviously isn't true.) But despite the fact that I am a structural engineer he might know concepts about structural engineering which I would have never heard of! Also he earns nearly 10 times of what I earn. But he doesn't belittle my achievements.
I have been on the receiving end of some scolding and a look of utter surprise for not knowing what the Ramanujan number is!
Even after years there are some of his achievements which were still unknown to me. Be it winning some national level competition or about him knowing some 10 different languages. I discovered some while casually browsing his laptop and reading his resume!
He has ZERO dressing sense. I have to be ready to be seen in public with someone who has oiled his hair and at times worn black trousers with black tee or a formal shirt with cargos.
No sense of cleanliness. Also he doesn't mind cats, dogs, monkeys, spiders, lizards living with him in his hostel room. :P
People sometimes consider him as an idiot for having selecting such a dumb girlfriend!
Even after years I haven't received any gift because he feels such trivial things are a waste of time! :D
I can be carefree when I go out anywhere with him. He has a habit of reading and remembering items on menu list with prices in restaurants. He reads all the boards on roads, so if I want to go to some xyz park he will remember that the park opens at 4 pm and hence take me there after 4 pm.
One incident:
Stranger: Where is Kamladevi Hospital?
Him: There in that small lane.
Me: Isn't that Mittal Hospital?
Him: It is popularly known as Mittal Hospital but I know that it is Kamaladevi Gauridas Mittal Hospital! :D :D
EDIT:
Me: Wow just have a look at my answer... It is getting a lot of upvotes.
Him: The other day I tried editing it but I couldn't do it anonymously. Can you please do it on my behalf.
Me: What do you want to edit?
Him: It is Kamladevi Gauridutt Mittal hospital. You wrote it as Gauridas!
ME. :D :D :D
Meanwhile...
Me: Which is your favourite colour?
Him: I don't have any.
Me: How can one not have a favourite color! Which colour do you like more than others?
Him: I find all colours equal.
(This went on for more than an hour)
Him: OK, I lose. Can you give me some options.
Me: Orange, Red, Blue.
After 2 mins
Him: Orange it is.
ME: How did you choose orange?
Him: I won't tell you.
(After some argument)
Me: Tell me fast before one of us dies.
Him: I used random number generation theory... (With a big explanation.)
ME: :D :D :D :D :D :D
EDIT 2:
A few people here feel that it would be very difficult for me to adjust with a geek and some of them also feel sorry for me because he doesn't get me gifts and all. While initially it was very difficult for me, over the time we are becoming like each other. He has composed a few poems and gifted them to me. So we have stopped exchanging material gifts and we exchange geeky stuff. For example:
This is a birthday card that I made for him :)
And someone mentioned about dressing sense in the comments. He is free to wear whatever clothes he wishes to wear. When with me he need not worry about his looks. What's the point in being committed if even after years, I judge him by his clothes!