Do not offer to create a new customer account if a membership is required

src/pretix/presale/checkoutflow.py

@@ -254,6 +254,14 @@ class CustomerStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
             field.widget.is_required = False
         return f
 
+    @cached_property
+    def signup_allowed(self):
+        return not any(
+            p.item.require_membership or
+            (p.variation and p.variation.require_membership)
+            for p in self.positions
+        )
+
     @cached_property
     def guest_allowed(self):
         return not any(
@@ -330,6 +338,7 @@ class CustomerStep(QuestionsViewMixin, CartMixin, TemplateFlowStep):
             self.cart_session.get('customer_mode', 'login' if self.request.customer else '')
         )
         ctx['guest_allowed'] = self.guest_allowed
+        ctx['signup_allowed'] = self.signup_allowed
 
         if 'customer' in self.cart_session:
             try:

src/pretix/presale/templates/pretixpresale/event/checkout_customer.html

@@ -14,7 +14,7 @@
                         <h4 class="panel-title">
                             <input type="radio" name="customer_mode" value="login"
                                     data-parent="#customer"
-                                    {% if selected == "login" %}checked="checked"{% endif %}
+                                    {% if selected == "login" or not signup_allowed %}checked="checked"{% endif %}
                                     data-toggle="radiocollapse" data-target="#customer_login"/>
                             <strong>
                                 {% trans "Log in with a customer account" %}
@@ -23,7 +23,7 @@
                     </div>
                 </label>
                 <div id="customer_login"
-                        class="panel-collapse collapsed {% if selected == "login" %}in{% endif %}">
+                        class="panel-collapse collapsed {% if selected == "login" or not signup_allowed %}in{% endif %}">
                     <div class="panel-body form-horizontal">
                         {% if customer %}
                             <p>
@@ -65,6 +65,7 @@
                     </div>
                 </div>
             </div>
+            {% if signup_allowed %}
                 <div class="panel panel-default">
                     <label class="accordion-radio">
                         <div class="panel-heading">
@@ -93,6 +94,7 @@
                         </div>
                     </div>
                 </div>
+            {% endif %}
             {% if guest_allowed %}
                 <div class="panel panel-default">
                     <label class="accordion-radio">