The term "alternative lifestyle" or "alt lifestyle" covers a lot of relationship territory. Here's info on dating sites for most of the major alt lifestyles, including polyamorous, BDSM and lifestyles built around Goth and Metal music. Note that more than one alt lifestyle website has been hacked and its members extorted. When using these sites, exercise some caution and proceed at your own risk.
Polyamorous Dating Sites
Polyamorous dating sites provide a way for married persons to meet other married persons and initiate affairs. The most popular is the Ashley Madison website. You can become a member without charge; instead, you're charged each time you start a conversation with another member.
The benefits of the site are announced straightforwardly on the Ashley Madison homepage: "Life is short. Have an affair." There are also some potential hazards. The site has been widely hacked and some members extorted with the threat of notifying their spouses of the membership. Although Ashley Madison allows members to delete personal information, a 2015 investigation determined that after the deletion request the information was no longer publicly available but was still in the website's database (and therefore susceptible to being hacked).
Another popular polyamorous site is PolyMatchmaker, which caters to the "Poly and Ethical Non-Monogamist" communities. Signing up is free. After you've initiated or responded to an ad, however, there's a fee, with Premium options at higher fees. In order to use the site effectively, you'll probably need to choose a higher-cost option.
BDSM Dating Sites
BDSM dating sites cater to fetishists and those with an interest in erotic practices that include bondage, dominance, discipline, submission and masochism. Two popular sites are Alt.com and the larger BDSM.com. Signing up for either site is free; both sites charge different fees for activity on the sites.
EvilMonk is a BDSM-related website that discusses some of the dangers of using such sites along with advice about protecting yourself. One of the topics discussed, "Why Be Concerned About Safety?" cites some examples of meetings gone seriously and even fatally wrong.
Goth and Metal Dating Sites
MetalDating.com caters to users whose interest in Heavy Metal Music extends to a Metal lifestyle. Sign-ups are free as of this writing, although users are encouraged to contribute to the site when signing up.
GothicMatch.com caters to the Goth and emo communities. Signing up is free, along with a Standard Membership. As with some other sites that offer a free basic membership, full use of the site requires a Gold Membership for an annual fee. Note that the membership fee automatically renews unless cancelled.