Might I suggest you read some of the personal blogs?
I have a boyfriend, who I would not consider "secondary" to my husband. We all live together. All three families know all of us and are well aware of the situation. Of the four children, the first is mine from a previous relationship. The second is my husbands from a previous marriage. The third is my husband's and mine. The fourth is biologically my boyfriend and my's child. BUT-all four are being raised (fully aware of their biology) as my husband's and my children...
There are SO SO SO many ways to make things work out. The key is in allowing love to grow as it will.
Also-maybe reading some books?
Polyamory in the 21st Century just came out-I found it a very great book.
I'm just reading Divine Sex: Liberating Sex From Religious Tradition. It's about what the Bible ACTUALLY says about sex, what is and isn't a "sin". Very interesting and educational.
there are lots more books and articles linked on my blog (link to that is in my signature).
You're very "young" in this exploration. It's perfectly reasonable to have fears and concerns. The best method of dealing with fear and concern is to face it through research and education.