In the context of menstruation, CanImpregnate is called to see if the cum being added is fertile. In RJW, already being pregnant is enough to block a new one, but in menstruation it's fine if fertile cum enters a pregnant womb, or if cum goes into a different womb than the pregnant one. So never have a pawn with a womb be considered pregnant for those checks.

The other times that CanImpregnate is called is for the fetish, but that's already checked against DangerDay.

Also add some more null checks for the transpilers.

1.3/source/RJW_Menstruation/RJW_Menstruation/Patch/RJW_Patch.cs

@@ -122,6 +122,28 @@ namespace RJW_Menstruation
         }
     }
 
+    [HarmonyPatch(typeof(PregnancyHelper), nameof(PregnancyHelper.CanImpregnate))]
+    public static class CanImpregnate_Patch
+    {
+        private static bool PregnancyBlocksImpregnation(this Pawn pawn, bool _)
+        {
+            if (!Configurations.EnableAnimalCycle && pawn.IsAnimal()) return pawn.IsPregnant();
+            else if (pawn.GetMenstruationComps().Any()) return false;
+            else return pawn.IsPregnant();
+        }
+        private static readonly MethodInfo IsPregnant = AccessTools.Method(typeof(PawnExtensions), nameof(PawnExtensions.IsPregnant), new System.Type[] {typeof(Pawn), typeof(bool)});
+        public static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
+        {
+            if (IsPregnant == null) throw new System.InvalidOperationException("IsPregnant not found");
+            foreach(CodeInstruction instruction in instructions)
+            {
+                if (instruction.Calls(IsPregnant))
+                    yield return CodeInstruction.Call(typeof(CanImpregnate_Patch), nameof(PregnancyBlocksImpregnation));
+                else yield return instruction;
+            }
+        }
+    }
+
     [HarmonyPatch(typeof(Hediff_BasePregnancy), nameof(Hediff_BasePregnancy.PostBirth))]
     public static class RJW_Patch_PostBirth
     {
@@ -216,6 +238,7 @@ namespace RJW_Menstruation
         private static readonly FieldInfo MinimumFuckabilityToHookup = AccessTools.Field(typeof(RJWHookupSettings), nameof(RJWHookupSettings.MinimumFuckabilityToHookup));
         public static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
         {
+            if (MinimumFuckabilityToHookup == null) throw new System.InvalidOperationException("MinimumFuckabilityToHookup not found");
             bool first_fuckability = true;
             foreach (CodeInstruction instruction in instructions)
             {
@@ -251,6 +274,8 @@ namespace RJW_Menstruation
         private static readonly FieldInfo MinimumRelationshipToHookup = AccessTools.Field(typeof(RJWHookupSettings), nameof(RJWHookupSettings.MinimumRelationshipToHookup));
         public static IEnumerable<CodeInstruction> Transpiler(IEnumerable<CodeInstruction> instructions)
         {
+            if (MinimumAttractivenessToHookup == null) throw new System.InvalidOperationException("MinimumAttractivenessToHookup not found");
+            if (MinimumRelationshipToHookup == null) throw new System.InvalidOperationException("MinimumRelationshipToHookup not found");
             LocalBuilder pawn_index = null;
             // Like in the last one, we switch the arguments around for the second load
             bool first_attractiveness = true;