First comes love. Then comes marriage. Then comes an online bake sale to pay for an egg donor, a surrogate and in vitro fertilization. In the kitchen of their snug, rented loft off U Street, Will and Dan Neville-Rehbehn, a married gay couple, are “baking for babies,” as their Web site calls their efforts to raise the money they need to start a family . Tonight, they’re baking a Margarita Layer Cake that will sell for $70. “I need 12 cups of powered sugar!” Will, 30, calls out to Dan, 29, who is watching their red KitchenAid mixer whip tequila-lime buttercream frosting on high. “Got it,” responds Dan, double-checking the recipe on the couple’s iPad. The multilayered vanilla cake also calls for “margarita liquid cheesecake” — a filling made with agave-based 1800 Tequila Reserva Silver — and “roasted corn tortilla-chip crunch,” which are alternated in an 11-step assembly process.