The trio is among a number of celebrities backing the Say I Do drive by the Freedom To Marry group, which aims to legalise gay unions throughout America.
Martin Sheen, Eric McCormack, Rufus Wainwright and Melissa Etheridge have also signed the letter to the president, which calls on the politician to clarify his stance on same-sex weddings and bring an end to "exclusion from marriage" for homosexuals, according to the Los Angeles Times.
The letter reads, "Marriage is the coming together of two lives, marked by a public promise of love and responsibility in front of friends and family. And marriage brings not only public respect and personal significance, but also a safety net of legal protections, rights, and responsibilities for which there is no substitute."
Lesbian actress Lynch, who married her long term partner Dr. Lara Embry last year (10), writes, "My wife and I were married in May of 2010, and I can say without a doubt that marriage matters... As President Obama continues his journey toward recognising our right to equal taxation, protection and dignity under the law, I encourage him to listen to gay and lesbian couples and families so he can better understand how marriage equality affects us all."