Can you cast a creature with Bestow while you control a Steel Golem?
Under the current rules, you can bestow despite &quot;you can't cast creature spells.&quot;
The rules changed in the Magic Origins rules update to tweak the spell casting procedure and also to address the bestow interaction explicitly.
Matt Tabak (then the rules manager) writes:

I made some tweaks to the procedure of casting a spell or activating an ability. It basically boils down to the proposal of the spell, including making all necessary choices such as modes and targets. Then the game checks to see if what you've proposed is legal. Finally, assuming the spell is legal, you calculate and pay costs. In practice, not much is changing. Bestow gets a little easier to figure out. For example, if an effect stops you from casting creature spells, you can still cast one as an Aura. There are some strange cases with things like split cards or flash that become a bit clearer as well.

(emphasis mine) and

We've deleted [rule 601.5] that says you can't begin to cast a spell that's prohibited from being cast. The legality check now happens at one defined point, according to the changes to rule 601.2. [...]

The updated rules now include 702.102c which explicitly addresses this interaction:

702.102c. The check for whether a spell can legally be cast happens after its controller has chosen whether to pay its bestow cost; see rule 601.2e.
Example: Aether Storm is an enchantment with the ability &quot;Creature spells can’t be cast.&quot; This effect doesn't stop a creature card with bestow from being cast for its bestow cost because the spell is an Aura enchantment spell, not an enchantment creature spell, when the game checks whether the spell is illegal.