I am a shoplifter who has never stolen, an adulterer who has never had relations with another man's wife and a cocaine addict who has never touched the stuff.
Why do I say that? Because these are all parts of a possible potential direction in which I (or any person) COULD go.
So, you have an attraction to your chevrusa. I think it happens more than we talk about, but what should not happen is to ACT on it.
Jewish Law regards homosexual ACT as an abomination, but if you read sifrei halacha, it is clear that it happens, and has happened, in more times and places than most people ackowledge. And that people struggle with it in many settings and times. Why am I telling you this? Because we are given "fences," within the sifrei halacha, to keep us from stumbling. Look into those fences.
Keep your hands to yourself, etc. I am not saying it is easy, and if you really cannot control yourself, perhaps you need to find a new chevrusa or make sure you are only learning together in a safe place (i.e. no "yichud.").
I do not know how well you know your chevrusa and his family, but perhaps you are objectifying him. (How many men really know anything about the women in the pictures they look at? ) If you don't really know someone outside of the beit midrash, you only know part of him, and may be "fantasizing" about who he is and what kind of person, outside the beit midrash, that he is. Once you get to know him as a person you are much less likey to objectify him. Make him real to you, and he will (hopefully) become more to you than a "sex object," if you will pardon the term.
Is he married? Are you? Are you willing to destroy his family and yours?
I had a secular upbringing. I never did drugs because I feared getting caught up in the pleasure and the highs...addiction. "Do not trust yourself until the day you die." If you go too far, and you slip, the real question is, will you be able to live with yourself afterward? Will you be able to forgive yourself? Will you be able to live with the repercussions on yourself, your family, and everyone close to you? (And him and his?)
Thinking to go there once? Twice? Three times? A new lifestyle? It starts, like recovery, WITH ONE STEP. Take the step in the right direction (away) not the wrong direction.
We are created with a strong sexual drive, some more than others. To deny it is foolish, to not learn how to cope with it and control it can be very destructive.
First of all, do not deny it . It is there. Instead, deal with it. Seek out a therapist or someone you can not only trust, but really talk to CONFIDENTIALLY. And I mean STRICT confidence. Be wary of people within your community who may be asked, at some future time, about you or a family member, for a shidduch, etc. Instead, ask for refernces of good , qualified therapists outside your community (but be careful - interview your therapist. You do not want to end up in the "care" of someone who feels that it is OK, and acceptable to act on these or similar feelings. Be up front:ASK the therapist at your first meeting!)
These feelings can be part of normal sexual develpoment, and can pass in time. ( I do not know how old you are.) And they can spring up again. I do not have a good "frum" way of saying this, so please forgive me, but I will try. People say they are unnatural. If they were unnatural, would God have to forbid them? Say instead they are forbidden, like pork, lighting a fire on Shabbos, kidnapping, and idolatry. The feelings happen, they are real to you, to many people. It is our avodah to limit ourselves to those behaviors that are permitted, and not forbidden. It is our challenge as Jews, as God's creations.
Easier said than done; and I know it is a struggle, a difficult one, perhaps, for you.
To not acknowledge that would be unfair.
Seriously - be careful, put up some fences, and avoid situations in which the temptation to act could overwhelm you. Even people who "fall in love" in acceptable ways find that, over time, their feelings change. Lust fades, diminishes. Yours may change as your personal circumstances change in life.
Another thought: Your chevrusa could be a really great person. He could develop into a friend you would cherish for the rest of your life. I don't mean sexual, I mean a commited friend with whom you can share the ups and downs of life, support each other through "thick and thin," and rely on for decades. You may be risking something that could develop into a lifetime friendship of mutual support and appreciation. It sounds odd, but that is a different kind of "love," but it is not physical, but can be very rewarding (that is why there are so many books, movies and TV shows about "buddy relationships") Never underestimate the value of a true FRIEND. How you act could destroy that potential for both of you. Get to know him as a person, he will no longer be an object, and it may be easier for you to deal with and escape the strong pull of sexual feelings. (Hey, you may decide you don't like him at all!)
Be strong, be careful. If you need help, need support, need chizuk, go get some. You have your whol;e life ahead of you, and a few minutes af pleasure are not worth destroying everything you may have coming.
You owe it to yourself.
Chazak!