LGBTQ+ undergraduates have higher attrition from science and engineering (S&E) than straight and cisgender undergraduates and perceive that having LGBTQ+ instructors would benefit them. However, it is unknown how many S&E instructors are LGBTQ+, the extent to which they disclose this information to students, and how disclosure affects "all" students, both LGBTQ+ and non-LGBTQ+. In study I, we surveyed 108 LGBTQ+ S&E instructors across the U.S. to explore the extent to which they reveal their LGBTQ+ identities across professional contexts and why they reveal or conceal their identities to undergraduates. Overall, 75% of instructors came out to at least some colleagues but only 48% came out to any undergraduates. Instructors most commonly chose to conceal LGBTQ+ identities from undergraduates because they perceived their identities to be irrelevant to course content and anticipated negative student reactions. In study II, 666 introductory biology undergraduates were randomly assigned to evaluate one of two identical teaching demonstration videos except the instructor revealed her LGBTQ+ identity in one but not the other. We assessed differences in students' impressions of the instructor across conditions. We found no differences in most ratings of the instructor except participants reported higher rapport with the instructor when she came out.