"A seaside wedding could be devised/Me rumpled bedding legitimized" makes it clear that their relationship has become sexual ("Ooo, Mister Todd, I'm so happy, I could eat you up, I really could!"--it's not just the pie money that's making her so happy). Lovett would likely be the instigator, fulfilling a long-simmering desire ("there was a barber and his wife, and he was beautiful"). We can speculate on why and when Sweeney starts rumpling her bedding. I think it happens after the Act One Finale and before the start of Act Two. They become passionately linked by their mass-murder scheme in "A Little Priest", two sociopaths on a roll. Not being able to confide in or include anyone else, and finally allowed to be "alive at last and...full of joy", Sweeney becomes more complete and arguably more sexually awakened, making him receptive to Lovett's advances. He is also grateful to her for her ingenious, diabolical plan about meat pies. Perhaps that is also partly why he gives her the attention she has been seeking all along. SPOILER ALERT: He believes Lucy is dead, so he thinks he's not betraying his wife.