Occurs at Game Start Up;

Could not execute post-long-event action. Exception: System.MissingMethodException: void Verse.Dialog_MessageBox..ctor(Verse.TaggedString,string,System.Action,string,System.Action,string,bool,System.Action,System.Action)
at System.Runtime.CompilerServices.AsyncVoidMethodBuilder.Start[TStateMachine] (TStateMachine& stateMachine) [0x0002c] in <eae584ce26bc40229c1b1aa476bfa589>:0
at FluffyExperiment.Controller.<.ctor>b__1_0 () [0x0001f] in <609c25cece6f4bdea03413040193d7b3>:0
at Verse.LongEventHandler.ExecuteToExecuteWhenFinished () [0x0007c] in <00638943358e4807bd44cd7bc8f572da>:0
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
Verse.LongEventHandler:ExecuteToExecuteWhenFinished ()
Verse.LongEventHandler:UpdateCurrentAsynchronousEvent ()
Verse.LongEventHandler:LongEventsUpdate (bool&)
(wrapper dynamic-method) Verse.Root:Verse.Root.Update_Patch1 (Verse.Root)
Verse.Root_Entry:Update ()

Notes:

Pawns appear to have sex with appropriate indicators. Genitals are there. No indication of a Buff after the event.

Occurrence After Event, one for each participant;

Pawn Bob threw exception while executing toil's finish action (2), jobDriver=RimWorld.JobDriver_Lovin, job=Lovin (Job_63665) A=Thing_Human956 B=Thing_DoubleBed92384, toilIndex=3: System.MissingMethodException: bool RimWorld.RelationsUtility.IsDisfigured(Verse.Pawn)
at Fluffy_BirdsAndBees.JobDriver_Lovin_MakeNewToils_FinishAction.Prefix (RimWorld.JobDriver_Lovin __instance) [0x00046] in <561edd0ab41e41bbb8fed9c3ea169ed9>:0
at (wrapper dynamic-method) RimWorld.JobDriver_Lovin.RimWorld.JobDriver_Lovin.<MakeNewToils>b__11_4_Patch1(RimWorld.JobDriver_Lovin)
at Verse.AI.Toil.Cleanup (System.Int32 myIndex, Verse.AI.JobDriver jobDriver) [0x0001f] in <00638943358e4807bd44cd7bc8f572da>:0
UnityEngine.StackTraceUtility:ExtractStackTrace ()
Verse.Log:Error (string)
Verse.AI.Toil:Cleanup (int,Verse.AI.JobDriver)
Verse.AI.JobDriver:TryActuallyStartNextToil ()
Verse.AI.JobDriver:ReadyForNextToil ()
RimWorld.JobDriver_Lovin:<MakeNewToils>b__11_3 ()
Verse.AI.JobDriver:DriverTick ()
Verse.AI.Pawn_JobTracker:JobTrackerTick ()
Verse.Pawn:Tick ()
Verse.TickList:Tick ()

On Additional Observation it seems the Pawns do not feel like the Task was satisfied. They attempt to repeat the Lovin event multiple times in a rest period.