Living in a
manner that does not
conform to the normal
socially enforced stereotypes. Anything not consisting of a
nuclear family with
mother,
father, and
children is likely to be considered an "alternative lifestyle".
Same-sex relationships are definately considered as such, and so are
polyamorous groups and the like. Even some members of
minority religions may be considered such.
Unfortunately, it seems that at least some government agencies consider it a bad environment for raising children, denying people the right to adopt, ruling against them in child custody cases, and even occasionally removing the child from the home when the parent(s) are considered to be living an "alternative lifestyle".