1. It may or may not affect your friendship. I mean, probably it won't because you've been best buddies since Kindergarten.
But if either of you is hung up about nakedness or sex -- like if one of you thinks it is bad or wrong -- then it could cause a strain on your relationship for a while.
However, if you totally like each other and trust each other, and you both realize that bodies and sexuality are cool and good, then you should be okay.
Talk it over first to be sure you don't have issues of fear, or guilt, or religious shame, or huge anxiety that you might be gay.
If you are both happy, natural, and comfortable with being naked together, it should just be fun.
1A. You can't turn gay. It's like being right-handed, left-handed or ambidextrous. You are born wired either straight, gay, or bisexual. It's just who you are. If you two are just doing this to know what it feels like to share an orgasm with someone, and what it's like to be naked with someone, it won't turn you gay if you are not already gay.
However, if one or the other of you crushes on guys, falls in love with guys and fantasizes about guys when masturbating, well, this could be an experience that would switch your gay on high.
In other words, if you are straight and share orgasms, you'll like it a lot, but you won't suddenly fall in love with each other. But if you already have gay tendencies, one of you might fall in love. Hope that makes sense.
2. Only do what you are comfortable doing together.
If it's just about snuggling naked and practicing kissing, cool.
If it's about jerking each other off for the joy of orgasm, cool.
If it's about sucking each other off to know what oral feels like on both ends, cool.
Do NOT do anal together. That is much more complex and should be done only with a gay boyfriend you are in love with, because it hurts a lot the first few times and is a big commitment.
Stick to only snuggling, jerking or oral -- whatever you feel cool with.
Do NOT be embarrassed -- both of you are healthy guys, have penises that get big, and both of you come. That's just part of being a guy!
3. If neither of you have slept naked with someone and done oral or vaginal or anal sex, then you are both totally disease free and will remain totally disease free. If you're clean, you're clean! I mean, STDs don't just appear by magic. If you are both total virgins, and have never shared oral or intercourse with someone else, you *cannot* give each other an STD.
So it's up to you. Do you want to have some experience, just for fun and learning? Maybe to feel comfortable with your bodies and sex? Then that's totally cool.
Have a nice time! And feel free to laugh, or to be tender. You are two awesome guys -- be happy.