I'm a father of three boys. First off, I'll let you know some information about me. When I was a boy and an adolescent, my father never told me things about puberty, sex, or girls. He talked a little, but never told me what they were or what to expect. He never showed his body, except for when I was between three and five we'd take showers together. But later in my pubescent years, I became self-conscious about my body, and wondered if I was normal. My biggest concern during adolescence was if I was maturing properly. I never was exposed to seeing other guys nude, let alone my dad.
Then around 12 I became curious as to what other guys looked liked and started looking at nude pictures of men and boys. I stopped that not too long after I had started because I realized that was wrong of me and a sin. But while I was looking at that stuff, I justified what I was doing because I never was told what would happen to my body or what other people's bodies look like.
I'm not blaming my dad, I made those choices myself, but I do think because I was not informed or told, I had insecurities and underlying causes that led to that. I want to tell my boys about puberty, sex, and healthy relationships. I want to inform them and teach them from a Christian perspective. I love them and want what's best for them. I'm asking for help about how to tell them about their bodies, and how to handle them seeing me nude or even each other nude.
Right now, I still let my three year old son take showers and baths with me once or twice a week. But I stopped doing it with my other two sons when they reached five. I was unsure when to stop this, because I couldn't clearly remember when my dad stopped doing it with me, but I think it was around five. I've read that dad and sons should be comfortable with seeing each other nude, and that it actually helps with their self-body image and strengthens the father-son relationship. But I've also read that it could hurt them and their relationship and cause them to go down the path as homosexual. So I haven't let them see me naked since they were 5. But I don't think it would be good for me or for them to see me nude now that they're this old.
They are homeschooled, so they aren't exposed to gyms or seeing other guys. As far as I know, they've never seen another naked person other than me and themselves. I'm also wondering when to tell them about puberty, sex, and masturbation. I believe earlier is better, but I'd like your advice on that as well.
I home schooled my own children and I am actively involved in home schooling, even though my children are all adults now.
First off, while it should not be made an issue if you or your boys happen to see each other nude, I don't think it is good to be purposely exposed them to nudity. One of the failings in society is a loss of the idea of having a sense of shame. Paul once compared Christians to different parts of a human body. In his discussion, he made this comment: "And those members of the body which we think to be less honorable, on these we bestow greater honor; and our unpresentable parts have greater modesty, but our presentable parts have no need. But God composed the body, having given greater honor to that part which lacks it" (I Corinthians 12:23-24). There are parts of our bodies which can be openly displayed, but there are other parts which is not seemly to be seen in public. You teach this respect for their bodies and the bodies of others when they are young, so it becomes a habit for them. "Train up a child in the way he should go, and when he is old he will not depart from it" (Proverbs 22:6). For example, I have several times dealt with young men who understand that intercourse is wrong, but see nothing wrong with exposing themselves to their girlfriends. These are boys who did not grow up with a proper sense of shame. Set the habits in the home that you expect your sons to have outside your home.
In regards to teaching, the best is to answer any questions they have as they come up, but keep the information at the level they understand or need to know at that point in their life. For instance, you can start right away with the basic idea that a person's private areas need to be kept private.
If your eldest son is follow the typical development, he should already be in puberty, though I could not tell you how far. He is at the age that you should start talking to him about the fact that his body is going to change. See: When to Have that Little Talk. Most likely he will find the concept gross, especially if he has not yet experienced ejaculation. If he is at that point, cover the first three chapters in Growing Up in the Lord: A Study for Teenage Boys. (Count it as part of health class.) Don't be overly concerned if he doesn't get it. Leave the book around where he can read it if he wants. Wait until he hits his growth spurt and then ask him if he's ejaculated semen yet. If not, cover the first three chapters again, because they now have meaning to him. If he has ejaculated, then cover chapters four and five. Spend a small amount of time talking about how to clean up after he has an ejaculation and answer any questions he has. Make especially sure he understands that he is not to use computers or phones for viewing pornography and tell him why. When he gets to be around 16, go through the book again, but this time cover the rest of it.
Typically most boys don't like to talk about sexual issues with their parents, even though they badly want the information. The reason is that they have to live in the same house and they don't want to think of their parents as sexual or have their parents see them as being sexual. It is why I end up with so many guys writing to me -- because they don't know me and won't ever meet me.