Oh, you're free to exercise your religious faith all you want, but if you're going to be in business you need to know that not every customer who walks through your doors will be a Christian or a Muslim or a Catholic or a Buddhist, and as such, you should be prepared to address such customers should it NOT be well known and established that your business does not cater to the needs of those whose religious views do not mirror your own.
By establishing your business as a for-profit religious entity and marketing such as a religious entity, you head off most problems that could arise due to confusion as to who you are and what products or services your establishment provides.
In short, if I were a Muslim and I entered your Christian book store I'd know that all I'd ever find in your store were Christian books. Therefore, it would be foolish of me to think I could raise a fuss that you don't stock Muslim literature in your establishment because that's just not what you do. :shrug:
Of course, you're free not to go that route of establishing your faith-based business as a for-profit company, but you should know that in not doing so you leave yourself open to the possibility of someday dealing with a customer who may create problems for you on religious grounds. Are you ready for that?
The cake baker, for example, could still bake the cake but refuse to place a "groom-and-groom" cake topper on it. Provide the name and number of a vendor who will sell the topper to the gay couple and they can order it themselves.
The photographer could take pictures of a gay wedding, just leave them out of his professional portfolio.
The caterer could still prepare the food and even deliver same, but just not serve it.
There are compromises all over the place if people look hard enough. Problem here is people don't want to look. They're too stuck on "being forced" to do something they don't want to do, being fearful of possibly being sued or the possible fallout from doing business with gays and lesbians. Well, the only way anyone really knows is if you make a big deal out of it. Don't make a big deal of it. Just find other alternatives to meet the demand or politely refuse but recommend another establishment that may meet their needs. To me, that's just good business practice.