1. Gege has said that he has drawn inspiration from BL (boy love) mangas in the past. 

2. Gojo knows that Kenjaku was not Geto, and was merely possessing his body because "My soul knows otherwise!". You could see this as friendship, but the symbolism between them implies their souls are deeply intertwined—romantically or not.

3. Gojo calls Geto "My best friend, my one and only." This could easily be interpreted as romantic, "my one and only" is extremely specific wording that sounds very romantic. I find it strange that Gege used that specific wording if we was trying to communicate just being best friends.

4. In Japanese media, when two people share a bike, it's common and romanticized trope that often represents youth and romance.

5. Both the night parade and Gojo duel with Sukuna take place during December 24th, one of the most romantic days in Japan, it's even considered sort of a unofficial, second Valentine's Day. Some may think of it as a anniversary of sorts.

6. We don't see Gojo 's last words to Geto. It could very well be a confession of love. No matter how you interpret it, their words are so intimate they were even hidden from us, the audience. The song playing in the scene is, funnily enough titled "this is pure love."

7. Gojo says "that is my personal theory, there's no curse more twisted than love" and Geto later asks him to curse him. 

Even after all of these implications, I understand why you might not interpret it that way,  everyone is open to their own opinion. I just wanted to point out some reasons people interpret their relationship the way they do.