Most anti-discrimination laws have clauses like this one:
"Any person claiming to be aggrieved by an alleged discriminatory act based on race, sex, familial status, color, national origin, age, religion, marital status, sexual orientation, genetic testing, physical and/or mental disability is protected."
"2. It is unlawful for an owner or operator of a place of public accommodation to deny a person any of the accommodations, advantages, facilities, and privileges because of his/her race, creed, sex, age, color, national origin, marital status, or physical or mental disability."
(That's from
Maryland, BTW)
What this means:
if you have a business, you may not refuse service to black man, a woman, non-family member, an arab, a teenager, a Jew, an unmarried couple, a homosexual, or someone with a physical or mental disability.
This usually includes housing, so if you don't want to rent to that nice mixed race gay couple, don't put it up for rent.
Now, where are some legal lines?
I'm a web designer. I don't do porn sites, or gambling sites.
I also don't host them, because my data center has restrictions against such things.
I do not design or host sites that involve the sale of Tobacco products.
The short version is, laws surrounding those sites are complex and outside my area of specialty. I have a list of companies experienced in that industry that I refer the rare inquiry to.
I'm a photographer.
There is a divide between my hobby work and my paid work.
If you are paying me to shoot, that line is simple. "Is it legal?" and is it within my "areas of specialty". I can turn down wedding shoots, because I don't do them normally. I can't turn down something just because I don't like the subject.
If it's something for my own portfolio, I shoot what I like, and that's perfectly legal.