Assuming sexism is actually deism...
Based on etymology it is not 'obvious'. The word theism has a complex history of multiple, contradictory usages. For example, theism actually used to mean deism, but it has also been used to mean belief in a creator god as opposed to deism.
Per OED:
theism, n.1
Frequency (in current use):
Etymology: modern < Greek
θεός god + -ism suffix . Compare French
théisme (Voltaire).
(a) gen. Belief in a deity, or deities, as opposed to atheism. (b) Belief in one god, as opposed to polytheism or pantheism; = monotheism n. (c) Belief in the existence of God, with denial of revelation: = deism n.
(d) esp.Belief in one God as creator and supreme ruler of the universe, without denial of revelation: in this use distinguished from
deism.
As with most things on these issues, there is no ultimate 'correct' usage, just a series of personal preferences based on differing perspectives of what is most logical.