A man who responded to an online ad for sperm and now faces thousands of dollars in child support is living proof of the dangers of donating outside of a sperm bank.

"The biggest piece of advice is just to not do it," said Ashley Nicole Reeve, a Texas-based attorney practicing in family law and reproductive technology law. "I don't know that there is any sure way legally to protect yourself."

With reproductive technology advancing more quickly than the law that governs it, donors who go through less-than-official channels can find themselves in a gray area when it comes to child support.

"The medical technology really has gone further than what the law has protected at this point. People are using sperm donors and egg donors and surrogates more and more, and the law really hasn't caught up just yet," she said. 

Just ask William Marotta, a married 46-year-old mechanic from Topeka, Kansas, who is being asked by the state to pay $6,000 in child support after he donated sperm to a couple in his town.

Marotta had a contract absolving him of parental responsibility and says he has no contact with the child, but because he donated his sperm outside of a licensed institution, the state has gotten involved.

"No good deed goes unpunished," Marotta told TODAY. "With what I know now, I don't think I would have been the sperm donor."

Marotta replied to a Craigslist ad in 2009 from a local couple who said they wanted to find a sperm donor. They were offering $50. After discussing it with his wife, Marotta volunteered, turning down the cash Jennifer Schreiner and her partner, Angela Bauer, were offering in exchange.

Marotta signed a written agreement that relinquished him of parental rights and held him harmless "for any child support payments demanded of him by any other person or entity, public or private ... regardless of the circumstances or said demand,” according to The Topeka Capital-Journal.

Schreiner became pregnant with the sperm, and she and Bauer -- who are not married because Kansas does not recognize same-sex unions -- co-parented the baby girl. Child support only came up when the two women broke off their relationship, one of them got sick, and they applied for state services for the girl. The Kansas Department for Children and Families demanded they tell them the donor's name, which Schreiner and Bauer eventually gave, reluctantly.

The state filed a child-support claim for more than $6,000 against Marotta this past October.

"We never intended for him to financially support her. That was our responsibility as her two parents," Bauer told TODAY on Thursday. "I have the joy of raising her and loving her every single day and it’s because of William that I have that."

"This is not at all what we signed up for," Marotta's wife, Kimberly, told TODAY.

But according to the Kansas Department of Children and Families, it's exactly what they signed up for when Marotta artificially inseminated Schreiner in her house. 

“In cases where the parties do not go through a licensed physician or a clinic, there remains the question of who actually is the father of the child or children. In such cases, DCF is required by statue to establish paternity and then pursue child support from the non-custodial parent,” Angela de Rocha, director of communications, said in a statement. 

Ben Swinnen, whose Topeka firm will represent Marotta at his Jan. 8 hearing, said Marotta has no way to pay for the child support costs, never mind his unexpected legal fees.

"The cost of defending him is way beyond his means. The issue is way beyond him and the cost is way beyond his means. It goes much further than his particular case and it costs much more than he can afford," Swinnen said.

Swinnen said his client doesn't have any contact with the daughter who was born. As for the contract Marotta signed at the time of his donation, Swinnen said, "It appears to me like it was found on the Internet by the two women, but I cannot confirm that. It's just my assumption," he said. 

Sperm banks typically protect donors through state parenting shield laws, but less straightforward cases have arisen in the past:

•  In 2007, Ronnie Coleman of an Arlington, Texas, agreed to donate sperm to a sperm bank for a friend, but told her he didn't want to be the father to any child who was born, according the Texas Star-Telegram. Yet in 2008, the mother filed a paternity suit against him, forcing him to pay thousands in child support until an appeals court ruled four years later that even though the donor was known to the mother, she shouldn't force him to pay.
• In Vermont, a man who donated sperm to a female friend was required to pay child support because he maintained a relationship with the children. One of the mothers told The Associated Press in 2007, "Part of the decision came down because he was so involved with them. It wasn't that he went to the (sperm) bank and that was it. They called him Papa."
• In New York, a married doctor agreed to donate sperm to a young resident and her partner in the late 1980s, only to be asked 18 years later for child support, the New York Post reported. The court was able to nail him when they found he was sending money and cards to the child, which he would sign “Dad” or “Daddy.” The biological father’s name was also on the birth certificate.
• A Massachusetts court ruled this year that a Nigerian immigrant had to pay child support for twins conceived through artificial insemination a year after he and his wife had separated, the Patriot Ledger reported.
• But in Washington state, the Court of Appeals ruled in 2004 that a donor can’t be required to pay child support unless he and the mother have signed an explicit contract.

Swinnen admits his client should have thought through his decision a bit more before proceeding.

"Hindsight is 20/20. He could have consulted a lawyer, explored the legal implications," he said. 

NBC's Isolde Raftery contributed to this report.

More content from NBCNews.com:

• Sandy Hook students return: New principal, new building, old desks
• Cheered by colleagues, senator who suffered stroke takes 45 monumental steps
• Undocumented students can receive aid under new California Dream Act
• Video: Camera captures ATM smash-and-grab robbery
• The angels had NY accents, says senator who had stroke
• Stolen packages returned after thief is caught on video

Follow US news from NBCNews.com on Twitter and Facebook