Your question about the nature of the relationship between the two principal characters in Of Mice and Men suggests many speculations about John Steinbeck's thinking while he was still working on the plot for this novel. If he were to write a story about two men who were buddies and were planning to buy a little farm and live together, the reader might be entitled to suspect that they were gay, because straight men would want to get married and have children like all the millions of other couples in America and elsewhere all over the world. This consideration may have been just one of the reasons why Steinbeck decided to make one of the men mentally incompetent and in need of supervision. Then it would be a simple matter to invent an Aunt Clara who asked George to look after Lennie.
By making Lennie an imbecile, Steinbeck achieved a secondary gain. If you refer to the Introduction in the eNotes Study Guide, you will see that Steinbeck intended to adapt his book into a stage play, and the play actually came out in New York in 1937, the same year the book was published. Stage plays rely heavily on dialogue. Steinbeck's novella also relies heavily on dialogue, evidently in order to simplify the conversion into a play. (It should also be noted that the novella contains only two simple interior sets and no exterior sets except for a campfire scene which would be easy to simulate on a stage with a fake campfire and mostly surrounding darkness.)
Because of Lennie's mental handicap, it becomes necessary for George to explain everything to him with patience, and in the process George is explaining all the essential information in the story to the reader and to the future audience. One familiar example is the scenes in which George is telling Lennie about the dream they share:
"Guys like us, that work on ranches, are the loneliest guys in the world. They got no family. They don't belong no place. They come to a ranch an' work up a stake and then they go into town and blow their stake, and the first thing you know they're poundin' their tail on some other ranch. They ain't got nothing to look ahead to."
With Lennie's eager encouragement, George goes on to tell how they are going to own their own little farm and grow all their own food and raise animals, including the rabbits that Lennie especially loves to hear about. Most of the exposition in the novel comes by way of dialogue rather than straight prose, and most of the dialogue is between George and Lennie.
Because if George's caretaker role and Lennie's dependent role, it does not occur to most reader to suspect that there is any sexual element involved in the relationship, as could otherwise be the case if they were just two buddies of equal intelligence. Lennie displays a normal heterosexual interest in Curley's wife the moment he sees the flirtatious girl, and George expresses his normal heterosexuality when he talks about going to whorehouses. There is never any indication that these two men have any sexual attraction to each other.
I don't think that you could describe George and Lennie's relationship if Of Mice and Men as sexual. Nothing in the novel suggests that they have a relationship that represents a physical attraction. Instead, I would describe the relationship more like that of a caregiver to a patient or that of an older brother to a younger brother. In some ways, you could compare their relationship to that of the two brothers in The Rain Man. Lennie ability to reason and act according to sound reasoning skills is limited and therefore he is much like a child who has never learned the consequences that certain actions have. Because of his limited cognitive ability, he does not always act in ways that are acceptable in society. Therefore, George has been given the responsibility to care for Lennie and watch over him to make sure that he does not behave in an unacceptable manner. However, like a child who gets out of the site of his/her mother and draws all over the wall with permanent markers, when out of George's site or care, Lennie can't help but make childlike choices.